I’ll come out with an anti-recommendation: Don’t do GitLab.
They used to be quite good, but lately (as in the past two years or so) they’ve been putting things behind a licensing paywall.
Now if your company wants to pay for GitLab, then maybe consider it? But I’d probably look at some of the other options people have mentioned in this thread.
I would consider using your Synology for what it’s good at - storage.
My homelab has a Synology DS1618 and servers are Lenovo M90q systems. They have enough compute to get the job done, and use the Synology NFS mount for storage.