French gay slang, like its counterparts in English and other languages, can be pretty colorful and witty. But it’s also offensive and should be used with caution. Fortunately, Language Drops has a great article on how to say i'm gay in french (or soy gay) without offending anyone.
In English, people who are openly gay often use “gay” to describe their sexual orientation and identity. In French, it’s more formal to use the term homosexuel (“homosexual”).
When talking to someone who isn’t a male or female, you can address them with pronouns like elles and ils (pronounced as eh-lehs). However, the French language still has an old fashioned masculine-feminine distinction so some people feel uncomfortable using those pronouns when addressing non identified individuals.
If you want to be really casual, you can just tell them, “ta maman est gay” or “votre maman est gaie”. That will definitely get a smile out of them.
It’s a bit sexist to tell your friend that their mom is gay. If your friend doesn’t want you to know their parents are gay, it’s none of your business. I’m sure they would rather you not know their mums are lesbians or transexuals too. So save your friend the embarrassment, and let them decide what they want to share with you. If they’re comfortable, they’ll probably let you in on their secrets sooner or later.