In the English language gay is used mostly to refer to homosexuals, but it can also be applied to a group. It is a common term for organisations such as Parents, Families and Friends of Lesbians and Gays (PFLAG) and Children of Lesbians and Gays Everywhere (COLAGE).
The word has had some negative connotations in the past, but is now widely used in the same way as other terms like lesbian, bisexual, transgender, and intersex. Some people prefer not to use these more specific terms, and instead choose to simply say "gay".
It is still used as a slang word, but with less of the negative connotations. Gay is pronounced exactly the same as gai. The old fashioned usage to mean light-hearted or cheerful survives, and you can see it in the name of places such as the Gaiety Theatre in Dublin.
In French, you can't really say that someone is gay. However, it is acceptable to say that their mom is gay in French. The mother is usually addressed as ta mère, or ta maman in more formal situations. It is not considered a derogatory term, but it may elicit a giggle from some of the other family members.