How to change the default target branch for merges in Gitlab

you need master。 Then Project setting---Default Branch ---save change

Enjoy！

The default MR target depends on whether or not the repository is a GitLab fork.

Forks

If the repository is a GitLab fork, then the default MR target will be the default branch of the upstream repository. This relationship can be removed via the "Remove fork relationship" option on the Project settings page, after which the default MR target will be determined as normal for a non-fork repository (described below).

At time of writing, it is not possible to override the default MR target without removing the fork relationship, but that functionality has been requested in gitlab issue #14522.

Non-Forks

If the repository has no fork relationship, then the Default Branch setting on the Project settings page sets both (1) the default MR target, and (2) the HEAD reference of the repo on the GitLab server (which determines the branch that's checked out when the repo is cloned). Note that, due to a bug/quirk in git, problems can occur if a branch that was once the Default Branch is later deleted from GitLab.

At time of writing, it is not possible to change the default MR target independently of the Default Branch, but this functionality has been requested in gitlab issue #17909.