Describe the solution you'd like
Gitlab Provider should collect project variables having specific environment_scope (like test, ref, prod) and add the default project variables having "environment_scope=all"

What is the added value?
a) Gitlab provider integration would have a better alignment with Gitlab project variables/concept/ideas.
b) In this concrete use case the effort for managing project variables that are shared accross all environemnts is reduced by using "environment_scope=all".

Give us examples of the outcome

I suggest to support multiple values for "environment" field like:

...
spec:
  provider:
    gitlab:
      ...
      projectID: "453"
      environment: [ "ref", "all" ]
...

Or a boolean flag for including "all" (default) variables; something like:

...
spec:
  provider:
    gitlab:
      ...
      projectID: "453"
      environment: "ref"
      includeAllEnvironmentScope: true  # default false for backward compatibility
...

Observations (Constraints, Context, etc):
not that I am aware of

@domizei385 and @moolen may I address this topic to you ?! ;-)
What do you think about this requirement/idea ?