To clarify:
We can't create an gitlab issue in someone else's name. All issues will be reported by the same "fpc" account. All notes will be added by that account.
But, we can add a bit of text that says "this ... was created by mantis-user martin_f @martin_frb
Then if someone adds a new note, and want to refer to a previous, they know who wrote it, and can do "@martin_frb" if they need.
If we do not have the gitlab name, then we just use the mantis name, and there is no @martin_frb
As for assigning: the gitlab issues can be assigned to gitlab accounts during conversion. But here we only need the accounts of those few who have developer status in mantis.