This is pretty cool and one of the few things I couldn't find a viable alternative to in-house hosting so far. The only thing your "GitLab Runners as a Service" is missing is ISO 27001 (and optionally SOC 2) certification. That makes it again easier for your customer to get / maintain their ISO 27001.
(So far I used this cloud.init script to spin up and upgrade GitLab-Runner instants: https://gitlab.com/21analytics/gitlab-runner-cloud-init/-/bl...)