Very much YMMV, as Castle25 says at the top. IME it has been poached chicken or poached fish, nothing too offensive but dull and no choice. The other frustrating thing (to me at least) is I've found the GF is often also DF so butter for the GF bread alternative (sometimes a GF bread roll, sometimes rice cakes) is often margarine.

My general rule is the higher the cabin of travel the less likely I will be to book a special meal. In first class I'll never order a special meal as it will just be poached chicken served on a nice plate, in business it's a bit meh compared to what everyone else gets, in premium economy it's comparable, slightly embarrassed to say I've not actually flown long haul economy so I have no experience of GF food in that cabin.

I always try and take some nice GF crackers / biscuits with me so if I'm in First or CW I have cheese plates minus the crackers, close to 100 flights in CW and F and I must have asked crew on at least half of those, they've not said no yet.