The monkey4 engine is a subset (or superset depending on how you look at it) of the grim engine, also visible here:
https://github.com/residualvm/residualv ... ure.engine
I can't check at the moment, though the list command might not be including the GrimE variant.
The logic for this is here:
https://github.com/residualvm/residualv ... t.cpp#L142
It might be disregarding it when listing, though it is
If you're able to Add the game and run it, it's likely that you have the correct files.
I assume you followed the guide here
It might be good to check the data by running the step described here:
Also might be good to post a list of your directory here so we can take a look.
Finally, it's possible that no-one's done a Gentoo ebuild in a long time (I can see that the last daily
addition is from 6/7 years ago), so there might be Gentoo-specific issues.
EDIT: following a GitHub/Bitbucket rabbit hole, I found this here
regarding ebuilds, which might interest you. It looks to be more up to date.
EDIT2: Looking at it more closely, you'd probably want to add the following addition to Line 68