SSPL doesn't come up under free or open source licenses partly for this reason, actually.
This probably qualifies it as a service, but since it looks like gitlab doesn't install it or redistribute it Elastic isn't going to come after them (if they don't use SSPL code then SSPL doesn't apply). Plus, GitLab is effectively open-source, so they might meet the qualification for using SSPL anyways. Of course, I am not your/a lawyer.
According to Elastic, "it depends" - if you can just search by entering a term in the searchbox, you're probably fine; If you allow users to use Elastic's query language, you're very likely in violation. If you let your customers define Kibana dashboards, you're definitely in violation.
It's worth noting that you can do a custom deal with Elastic, as long as you pay whatever they ask & accept their terms (or renegotiate them, which is a fairly painful process) - so maybe GitLab did just that.
If this interpretation holds, I feel like it could be a pretty big problem for companies like GitLab, who not only have deep integration with ElasticSearch, but offer it as a premium-tier feature:
https://docs.gitlab.com/ee/integration/elasticsearch.html
Maybe this really is just a cash grab from the Elastic side, like, "it's too easy to self-host rather than paying for our SaaS offering, so now you need to pay us for many common self-hosting use cases also."