My wife is coeliac, which is why I've got into brewing - though I do more wine than beer. She works with somebody whose husband is a master brewer and claims all beer would meet the GF standard if they bothered to get it tested, barring those that have some malt added late on for extra flavour. OTOH, different people are sensitive to different degrees and I know of people who react to beer; and you can be asymptomatic for years then suddenly get serious neural problems, like my MiL, so my wife for one prefers not to risk non-certified beers.
Apart from the ones other people have listed, you might like to try the Spanish beer Estrella Damm Daura, available in Sainsbury.
All the certified GF stuff is expensive, though, so...
If you want to try brewing some, I can recommend the recipes by Duncan Disorderly over on the "Jim's beer kit" forum. The ingredients list is weird but the taste is great.