We've taken many GCT river cruises (and OAT trips and small ship cruises). We did both their Christmas Market cruises on the Rhine and on the Danube, but before Covid. We loved both of them. Yes, they're not as fancy as the more upscale lines, but they focus more on "people to people" activities. This will include going to local churches for Christmas choral events, home-hosted meals as well as going to the Christmas Markets at each stop.
Dinner always had three options (meat, poultry, fish) usually with a vegetarian option. If you have any food issues, let them know, and the chef will speak with you each night to set up the next night's meal. Of course, you'll have a soup, salad, side dishes, dessert, etc. I am somewhat picky but always found something I liked. Included wine selections vary in quality, but you do have the option to purchase more "upscale" wines if you wish.
We had actually planned on doing a Christmas Market cruise (a repeat) this coming December, so we recently looked at all their options. However, we decided to do one of their Egypt trips in December instead. The world has too many places to visit!