Hal Fischer didn't realise he was making history when he took the pictures that appear in Gay Semiotics and Other Works, which opens at the Gallery of Modern Art in Glasgow next month. He was too busy living through it. As a gay man in his twenties, who breezed into a post-hippy but still sexually liberated San Francisco in 1975 to study photography, he embraced the scene he landed in with relish.

This comes across in the twenty-four photographs that make up Gay Semiotics, which, on one level, capture an array of cock-sure young men who look like they've stepped straight from the pages of Armistead Maupin's Tales of the City stories then being serialised in the San Francisco Chronicle newspaper.

Taking things further, as the title of the series hints at, each image is accompanied by a text that explains its pictorial iconography with deadpan pseudo-seriousness. The effect is of an in-crowd pastiche of some socio-anthropological textbook that might just allow straight society to get a handle on the signs and signifiers of a wild life elsewhere.

This includes identifying the meaning of how a set of keys or an earring dangles, or which denim butt-cheek pocket a particular coloured handkerchief should hang from to indicate a sub/dom preference in a world of mustachioed clones. The result is a gleeful portrait of what, in a post-Stonewall, pre-AIDS era, was both a more innocent and more liberated time for gay men.

'I came out in a place and a time where it all felt very natural, and that comes out in the work,' says Fischer today. 'The pictures aren't exploitation. This was my world at the time, which I'm sharing, and that's a kind of liberation in itself, and it was without consequences. There may have been a naiveté there, but the work reflects the world I was part of at that time, and I'm celebrating that.'

Gay Semiotics caused considerable waves when it first appeared in 1977, its presentation of gay sub-culture in conceptual form giving the work a swagger that broke the mould of mere documentation.

'It came somewhere between photography and conceptual art,' says Fischer, 'and that made it unique. Robert Mapplethorpe was doing stuff, and right from the very first review of my work Robert and I were being set up and spoken about together. I knew Robert, but I was doing something different to him, and I wasn't trying to be voyeuristic.'

Also included in the GOMA exhibition are 'Boyfriends' and '18th Near Castro St', two other series' in which text and image co-exist on equal footing. 'Boyfriends' presents ten portraits of men of Fischer's acquaintance, with a paragraph on each outlining their usually short-lived liaison. Shot over twenty-fours, '18th Near Castro St' shows the life in a day of a bus stop bench in the then-thriving Castro district.

'Castro was unique,' says Fischer. 'I remember going there the first time, and I didn't know what was gay about it, but then you just had people hanging out on the street in this gay village.'

Where Gay Semiotics was gleefully of the moment when it first appeared, more than forty years on it has become a period piece that looks like another world. Part of the changes documented in Fischer's essay came following the murder in 1978 of politician and Castro resident Harvey Milk, who was the first openly gay elected official in California. Once AIDS arrived a couple of years later, the party was well and truly over.

Fischer documents these changes in At the Centre of the Gay Universe, a new essay that features in The Gay Seventies, a just published volume that features all the images in the GOMA show and more. The essay also touches on a more complex and diverse history than the very personal experience depicted in Fischer's photographs.

'I'll be honest,' he says, 'one of the things that worried me, and discomforted me, when all this started happening, was that I did this thing about white gay men. Were all the politically correct gang going to come at me and start asking, where are all the lesbians, trans people and people of colour? But you know what? People didn't care. They recognised that this was my experience, and in that way it was autobiographical. It's definitely a portrait of me.'

As the anything-goes optimism of the 1970s moved into the 1980s, Fischer gave up photography and journalism for a museum career.

'I was an art writer at the time,' he says, 'and I found myself doing more and more writing. I was never really that prolific in my photography. The writing superceded it, and I became less interested in photography. I also thought that after I'd taken these photographs that I'd brought things to a kind of completion and realised, intuitively or not, that the instability of a freelance writer or photographer wasn't for me.'

Between 1985 and 2007, Fischer was director of exhibitions and publications at the Timken Museum of Art, and directed special projects at the Asian Art Museum of San Francisco.

'I had always been fascinated by museums,' he says, 'and once I went into that world, a lot of it was about how you explain art to people, and I was really happy doing that. It was creative.

'I look back now, and I think, what would I have done after taking these pictures? It's with a sense of relief as well that I wasn't a photographer during the AIDS crisis. The critic in me looked at my work and said, you're never going to do this again, so I'm glad I stopped when I stopped.

'In the early 80s as well, there was a new way of doing things that became incredibly bloated, and I'm just not a PR person. In the 70s, no one had any money, but people went to openings and discussed things. In the 80s it all became much more calculated.'

This hasn't stopped the new wave of attention Gay Semiotics has had long before its Glasgow showing, which began when Fischer's images were included in Under the Big Black Sun: California Art, 1974-81 at the Museum of Contemporary Art, Los Angeles. The work was subsequently shown in several solo and group exhibitions, including a London presentation in 2017.

'I never saw any of this coming,' says Fischer. 'All of these kids in their twenties come up to me, and they know the work.'

What, then, is the response to Gay Semiotics across the generational divide?

'People of my generation who are still here look at it with a certain nostalgia,' according to Fischer, 'because that whole thing that was fun disappeared completely. The kids in their twenties who see it, on the other hand, look at it and think we were so lucky. But then, I remember when I was a young gay man in my twenties, and that was the time when everyone came out, and a lot of older gay men lost a lot of camaraderie because of that, and it became a whole different thing. Certainly it was a very different sense of community, and that was what I think they were responding to.

'But I see people looking at those pictures now and they're laughing at them. They can see so much of the 70s in the work, but the fact that the humour in the work is still there and still packs a punch, that means a lot. I think people see an integrity and a certain authenticity there. In those pictures I'm talking about me, and people see that.'

Hal Fischer – Gay Semiotics and Other Works, Gallery of Modern Art, Glasgow, Fri 15 Nov–Sat 30 May 2020. Hal Fischer: The Gay Seventies is published by Gallery 16 Editions.