The SMB2 leases capability is determined at the negprot stage whereas the the check for gpfs:leases is done in vfs_gpfs at the tcon stage.
As the latter enables kernel oplocks if gpfs:leases this results in an incompatible runtime config: SMB2 leases enabled and kernel oplocks enabled.
The following messages are an indication for this:
linux_release_kernel_oplock: Error when removing kernel oplock on file ...
Error was Resource temporarily unavailable
I think we need a check in vfs_gpfs that detects this incompatible setting, log an descriptive error message and deny access.