@Revanth Hi and welcome to the Forum 
General support for SLES 15 SP5 ended 31 Dec 2024 you can try but doubt the repositories are online unless you had an LTSS subscription… LTSS support ends 31 Dec 2027.
The registration code should work with any SLES release, why not SLES 15 SP7?
Adding the Workstation extension module should give you the ability to install the GNOME desktop environment.