Merge Requests dependencies
The procedure when submitting new changes, is to create a new issue in bugzilla and attach the patch(es) to it. Sometimes it took a while for that patch to be reviewed, and it happened that you could work on new changes using that same modified tree. If those new changes were related to the original issue, it made sense to add the new patch(es) to that same issue. Otherwise, the most sensible thing to do would be to create a new issue, add the new patch(es) to it, and relate this new issue with the first one using the Depends and Blocks fields.
I believe that the new procedure would make use of the Merge Requests (MR), which eases the submitting procedure. Going back to the previous procedure, once new changes are submitted by using a MR, if new changes occur, and they happen to be related to the MR, they will be added automatically, as commits made in the same branch will be included in the MR (those change can even be squashed into a single commit).
However, in unrelated changes, it would make sense to create a new MR, which would depend on the original MR, in a similar way to the Depends and Blocks fields of bugzilla, but at the moment this does not seem to be possible.
There is an opened issue about this in GitLab.