Typical message looks like:
Merge branch ‘mass-refactoring’ into ‘master’
Mass refactoring
See merge request group/project!13
Is it possible to change template to something like: ?
HEADER
BODY
Merge branch SRC_BRANCH into TGT_BRANCH. See merge-request GROUP/PROJECT!N
As listed here (gitlab issue #2551) the wish for such a feature is still present (4 weeks ago).
To answer your question, no, there is none built-in gitlab feature to change the merge message template.