Works for me, I've been using Gboard since I got it. Is it selected under Settings -> More Settings -> Language and Input -> Current?
And clearly I'm not the only one impacted, e.g. https://old.reddit.com/r/Onyx_Boox/comments/1hoh143/gboard_w...
I often need to open Gboard app, and it'll present the "step 2" of the first run experience when it's installed by asking me to select an input method, at that time Gboard would show up as one of the installed input methods.