All secure boot does is ensue the binary (say, Linux or Windows kernel) run in early boot is "trusted," meaning it's cryptographically signed by a key the motherboard has. You can usually load your own keys and sign your own binaries, but I imagine only large orgs do that if they have a lot of Linux systems or something.

The way Linux works with this is they use a shim binary that is signed by Microsoft's key, and that binary loads the actual Linux kernel. The kernel itself is not signed with that key.

The only way this impacts gaming is if games check if Secure Boot is enabled. If it is enabled, the game knows the system booted with something signed by a key the motherboard trusts. For most systems, that means Microsoft's keys, but AFAIK, they can't check what key was used in early boot unless the kernel provides some indication of that.

Basically, it's an anti-tampering check, so they have some assurance the kernel is untampered from what the maintainer released.