Hi and many thanks for the support!
Indeed, GitLab does not provide any variable for the trailer.
To overcome this, I've created a template that includes the pattern with all possible values. E. g:
`Changelog: added | changed | removed | whatever`
Everytime I create a MR, I just keep the appropriate trailer. It's not super convenient but it avoids to rewrite the whole thing and makes sure you don't make a mistake with the wordings.