Hiya Komgrit
I believe the Gitlab plugin is supposed to support use of GoCD personal access tokens (at least there is code that handles it) .... or was at some point with some particular Gitlab version.
If it's not possible to get it working via the Gitlab plugin for gitlab users, you'd need to enable an additional authentication/authorization configuration (e.g using the bundled passwordfile plugin, create some kind of user for use by your automation, and either use the username/password with that user via the GoCD API or generate a personal access token tied to that user).
But probably better to figure out why the Gitlab plugin isn't working first, as I imagine you'd rather not add an additional identity source if you can avoid it.
-Chad