Every time I use GitHub I am supporting proprietary software and silo systems.
Every time I use GitLab I am supporting open source software and open systems.
GitHub is a closed silo — you can do with it only what the company chooses to let you do. It’s like using WhatsApp instead of Matrix, or like using Facebook instead of email. They can start placing annoying adverts, restrict what external systems you can integrate with, introduce charges for services that were once free, and change their terms of use in any way they like at any time.
I don’t see a FOSS project’s values being upheld by using GitHub. Yes, it provides a useful service, for “free” (as in beer). But you get sucked in, then you get locked in.
GitLab, by contrast, is open — Free as in Freedom.
By using GitLab, in addition to supporting FOSS, we can adapt it to fit our needs and desires. Use our own hostnames (gitlab.MYPROJECT.org). Integrate with our own user accounts. Own our own data. Our own project mirroring configurations. We can choose to host and run it ourselves, or get gitlab.com or another provider to do so, with freedom to migrate back and forth. And no lock-in to the vendor’s policies and charges.
If you do open source software, put it on GitLab!
(I have no connections with the companies/products involved, just a desire for software Freedom.)