Often when products are released they lack the required polish with certain, crucial aspects missing. Gboard dictionaries are a handy feature and one such example, where one can have a global dictionary and other, per language entries.
Gboard often places custom keywords (entries) in the wrong places (other than the intended dictionary) and it's up to you, the user, to correct that mistake. It likely guesses the input, not places it based on the selected language you're currently typing in.
This becomes an even bigger challenge with multilingual typing.
The missing aspect
The greater issue is that Gboard currently doesn't have a feature that would allow you to select multiple entries and move them to another dictionary.
The proposed solution should cut down entry migration time by half, at least, since it no longer requires too much manual labor (remove entry, navigate to next dictionary, re-add entry from scratch).
Try out the prototype. It's a little rough, but enough to get the idea through, and while not fully optimized for touch gestures on the phone it should work on desktop using the mouse.