I did not fly MLE-IST or IST-ZRH and do not know the exact meal they will serve on those flights during your days of travel, but I also order GFMLs all the time. On more than 10 TK flights in J during the last four years, those GFMLs have always been awesome (or at least good, one was ok).
TK, OS and SQ meals beat every other carrier I've flown (>15 different ones).
The GFML they serve is usually so good I would even order and enjoy it in a restaurant on the ground.
With TK GMFL, they always served a nice starter plate with something like "turkish antipasti" (whatever the correct name might be
). Sometimes this comes with rice waffles, other times you'll get a gluten free bread roll (usually served later with your main course if you don't ask to have it served earlier).
The main courses I had have usually been chicken, fish or lamb, served with rice or potatoes, and different kinds of vegetables.
Your wife can absolutely plan on eating the GFML and even look forward to it!
The only thing I didn't like is that I never was served real butter, but some kind of diet replacement thing instead. Everything else always tasted great.
Just watch out for the FAs walking around with small plates of starters (the same size like small plates of nuts): 3 out of 10 times I was offered something by the FAs that looked like small pieces of bread with nice appetizers on top. So I always asked "Excuse me, I have some allergies so I ordered a GFML. Is this gluten free?". They asked the chef, and everytime I suspected it wasn't gluten free, it really wasn't... In J you have a chef / cook (they are not FAs, but real catering professionals employed by DO&CO - not by TK - I talked to some of them). They serve long range J cabins and also IST-ZRH if you have real J seats (not Y seats with free middle seat - still a nice GFML then, just no chef on board).
Whatever you receive on your personal, "bigger plate"/tablet is 100% GFML and whatever the chef serves to you is 100% GFML (if you ordered it). So just make sure when the other FAs walk around with their small plates of appetizers/desserts they hand out to everyone to deny that or ask if it is gluten free.
Enjoy your flight!
Cheers,
zap