How do you gain friendships as an autistic young adult?

Not really sure but my main advice to you would be, be cautious. It's better to be alone than to be in bad company. I'm an alcoholic so people have often seen me with a bottle on the streets. As a result, some unpleasant people started talking to me. I got pulled into things I didn't want to be. Never, ever talk to beggars or people who look common and rough. It's better to be alone than in bad company - I will say it again. Dress smart, ask the women in your family for advice on fashion, eat well, be in good shape, have an interesting appearance and be as happy as you can, pursuing the things you like, and with good luck people will approach you.

This comes up a lot on the forum. Try something like this: https://www.meetup.com/.

Friendship isn't just about people having the exact some interests and hobbies you can connect on so many other levels. If you've got esoteric interests you'll just have to broaden out and connect with people away from that. 

'Behold my son the allistic man

And be as like him as you can.

He talks a lot, but his mind is slow

You must conform, to be his amigo.'

Apologies to the poet Hillaire Belloc.

You find other ASD people in groups about your interests.