I'm not sure about gift cards, but care packages were appreciated by the soldiers (usually if it was crap they did not need, they would give it to the Iraqi Army, or Ugandans who would greatly appreciate it).
As far as holidays, most soldiers get sick of the holiday decor in the chow hall, and at the MWR, and the damn christmas music they play non-stop at mealtimes. It reminds people that it's the holidays, a time people like to spend with their families and they are as far away from their families as possible.
I'm not knocking the idea, I think it's great, and I'm sure Xerox will get some tax right off for this "charity", but I'll stick with sending boxes filled with "Listerine" to the Marines I know over there.
[EDIT]
I misread the first post.
I thought these were holiday cards for some reason. As Thank You cards... Yeah, great idea!
[/EDIT]