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The only reference to GitLab versions I can see is this one: https://github.com/gitlabhq/terraform-provider-gitlab/blob/master/CHANGELOG.md#100-october-06-2017

1.0.0 (October 06, 2017)

BACKWARDS INCOMPATIBILITIES:

This provider now uses the v4 api. It means that if you set up a custom API url, you need to update it to use the /api/v4 url. As a side effect, we no longer support Gitlab < 9.0. (#20)

Thank you @little-cartoon-10569!