Update: I bought both of the linked covers.
I like the KDBFans 60% acrylic cover since it fits the GMMK Compact very well. The exterior dimensions almost perfectly match the keyboard case dimensions. There is room inside the cover to keep it from pressing any keys or scratching the sides of the keycaps. This one gets a 
from me for the GMMK Compact.
The mStone TKL acrylic cover does not fit the GMMK TKL very well. The cover is too large in both the length and width. mStone includes a silicone spacer (see pictures below) that you can put on the edges that gives the cover just enough interior width to sit on the keyboard case. It will protect your keyboard, but it definitely looks too big for the GMMK TKL. You can see the top-bottom overhang in the top view photo below and you can see the side overhang in the last photo.
If the KBDFans TKL cover ever gets restocked, I’ll buy one and test it on the GMMK TKL. As always, if anyone still has recommendations let me know.