The most popular snorkelling points off the beach on Trawangan are along the east and north coast, with the best time to go being an hour or so at each side of high tide.
Visibility can be exceptionally clear.
The coral beds in most cases are in a devastated state. You'll see plenty of fish, but the coral really is a mess. There is evidence of some restorative work, with Biorock frameworks set up in some areas (as in Pemuteran in western Bali) to encourage coral regrowth, but it is very early days. That the boatmen continue to toss anchors willy nilly rather than use buoys (when available) doesn't herald much hope.
We'd heard very mixed reports on the state of the coral off Gili Trawangan, but they were in fact worse than we expected. The "best" that we saw was off the north coast, more or less out from the area between Alam Gili and Beachcomber.
The one good aspect of snorkelling here is that you'll have a good chance to see turtles. The best area for snorkelling with turtles is off the northeast coast, roughly around Blue Star Bungalows. There's a steep drop-off offshore and if you're in luck you'll see a turtle or two.
There is no need to bring your own gear as snorkelling equipment is widely available for hire, both from freestanding hire shops and most guesthouses. It can also be purchased from shops. The standard is generally pretty good -- while the reefs may not be getting much care, the gear certainly is.