experience of/in

More or less. But the whole sentence reads badly.

Try

John's first experience of participating in the US Gay Pride movement (celebrations? parade?) made him astonished at the (many, real, everyday, acute) predicaments/ problems that homosexuals face.

John's first experience participating in US Gay Pride astonished him at the very worldly predicament the homosexual faces

Thank you Tctag, but for dalehileman, why

-the homosexual faceS

?

The old, the young, the rich, etc. are plural; or is it a typo?

That's really a good Q, WB, in fact one I had asked myself as I was composing my response

No no no, Dale, you haven't answered my question; do you think in American English 'the homosexual', 'the rich', etc. are both singular and plural?

I'd say so WB but don't quote me

Thanks for supporting me, but today my friend (non-native) saddened me by telling me that we could say

- a doctor with experience in (not OF) dealing with patients

and that I was wrong; do you think the sentence above is a different structure?

- a doctor with experience in dealing with patients
- a doctor with experience of dealing with patients

To me, there is no real difference here between those two. Both equally grammatical, both meaning the same.
In deference to your friend, I guess that the first might be slightly more commonly seen.

If I were to differentiate between the two, I'd use a different example: IN meaning in a particular field (such as geriatrics) and OF meaning of a particular problem (such as hernia).
But that is a fine distinction.

Mac I'm impressed by that distinction. I had a feeling there was a slight diff in meaning but couldn't express it

Thank you, kind sir. I'm sorry if I was rude to you yeaterday. Sometimes I get a bit frustrated.

Quite welcome

Oh Mac quite all right. Contrast with S. in other recent posrtings

Don't we all

Thank you Mctag.