That's a Micro Four Thirds camera, so you've actually got a fuck ton of options for lenses at very reasonable prices, even brand new. Have a look around for what Micro Four Thirds lenses are compatible with the GH5 (it may well be all of them — I'm not too familiar with that system, but I'm fairly certain one of the big selling points is that you can use lenses by pretty much any manufacturer for that system). If you wanna go even cheaper, jump pon the bay.
Other than that, you could get an old Helios M42 mount lens and then get an M42 to Micro Four Thirds adapter. This can be risky thoug