In short
The three Gilis — Trawangan, Meno, Air — ban motor vehicles. You get around on foot, by bike or by cidomo, the horse cart.
Green and hawksbill turtles are met while snorkelling straight from the beach, with no boat and no excursion. That is the main reason to come.
What it really costs
The Gilis cost less than Bali for comparable accommodation, but everything is shipped in and food prices show it.
Each island has its character and its price: Trawangan is the liveliest and dearest, Meno the quietest, Air in between.
What nobody tells you
There is no fresh water on the islands: shower water is brackish and drinking water is bought. That is not a fault of the accommodation.
Fast boats from Bali have had accidents and not all operators are equal. Checking life jackets and avoiding crossings in heavy seas is ordinary precaution here.
Cidomos are a contested subject: the treatment of the horses is regularly criticised by local associations. Walking or cycling works everywhere — the islands are a few kilometres across.