If conflicts occur during a rebase or a merge, git will stop and you will have to fix them.
use git status to list the files with conflicts
fix the conflicts and add the fixed files to the index with git add
when no conflict remains, resume the merge (with git merge --continue) or rebase (with git rebase --continue) operation.
if you prefer to cancel the operation instead of resolving conflicts, use git merge --abort or git rebase --abort.
(For Gmsh "masters") Accept a merge request
First check that the commits in the branch do not break the testcases (the gitlab server should put a green icon beside the commit and on the page of the merge request) and that the code looks good.
If it's not the case, you can discuss the modifications on the page of the merge request on gitlab and ask for changes. Once the branch is satisfactory, merge it into master.
You can do the merge right from gitlab's interface, or on the command line: