Hey guys, apologies for a bit late response.
Anyway, the issue with ruby exceptions is solved now in master branch.
There will not be a new release just yet, but you can pull that source and try it if you like.
As for your problem with building mixed kind of projects (with branch parameter and without it), there should be no problems. Namely, the ruby exceptions issue was stopping the plugin from processing other jobs so that should no longer be a show stopper.
The other problem you seem to have is related to GIT_BRANCH parameter which is not of a String type. The idea behind this parameter is that the plugin will fill that parameter value with branch received from Gitlab payload. If I understood, you have jobs which have GIT_BRANCH parameter defined in Source Code Management --> Branch specifier (in job definition) and the parameter is a choice parameter (with predefined values). If that is correct, I've also added a fix (in master) that will skip such projects.
I'll close the issue here, but please feel free to open it up again (or any other) at project's Github page. Thank you!