There is an updated pull request by ericonr against master on the void-packages repository

https://github.com/ericonr/void-packages master
https://github.com/void-linux/void-packages/pull/18291

gtkmm: change version number to v3
The short description for this package was wrong, as it was using v2 as
the version number.

Fixes #18290 

A patch file from https://github.com/void-linux/void-packages/pull/18291.patch is attached