But according to Red Hat in the interview linked below all of RHEL is built from CentOS Stream, is having the source code available in CentOS Stream Gitlab not adhering to GPL ?
Nope. Centos Steam is merely upstream of RHEL. GPL stipulates that when you give someone else a binary, you also give them the source to that binary, not something similar.
Isn't that the whole issue here? Customers and people with Developer licenses can get the exact RHEL binaries "behind the paywall". And even then if something is upstream does that not mean that the same code flows down stream?
No. CentOS stream is irrelevant here. RHEL customers cannot meaningfully distribute the sources of RHEL. This has been the issue since the beginning, it's just that RH has tightened the knobs progressively over the years.
RHEL source according to Red Hat is CentOS Stream. If the source of RHEL is CentOS Stream then how is it irrelevant? Red Hat cannot deprive you of your right to sources and to redistribute them under the GPL. But Red Hat can also determine who they want to do business with.