We're new to Go on z/OS and I'm wondering if the Go 1.20.5 package for z/OS is a straight port of the Go package, or if it's a proprietary IBM invention of some sort?

If it's a straight port, has the source code for z/OS been integrated back into the public  Github project? Could a customer wanting to build his own version of Go build it himself, or is he dependent on IBM for ongoing updates? Is this build process documented somewhere? 

We have the old Go 1.6.4 implementation running on z/OS and that seems to be built from the published open-source master for that release...so far, we haven't found a way to do this same thing with the 1.20.5 package. 

Go on z/OS is a port to z/OS of the upstream Go compiler. It is licensed as open source, but it isn't on the google download site. You can get it easily though, either as a PAX download or with SMP/E (and optionally you can pay for service). Here's the product page: https://www.ibm.com/products/open-enterprise-sdk-go-zos It isn't anticipated that you'll build it yourself (or need to, really). We work to keep the z/OS version current. There are also upstreamed z/OS compatible versions of extended libraries x/sys, x/net, x/crypto, etc. Is there a particular reason why you wish to build the Go compiler it yourself?