Hello Jhon Michael Bujawe
If you have a GGWA license with downgrade rights, if Windows 10 Pro is already installed on your PC, have you tried entering the GGWA product key to activate that license?
As far as I know, if it was acquired under a Microsoft Volume License Program, it would be entitled to downgrade rights. Windows 10 Pro VL under GGWA is basically a full license.
Exercising downgrade rights would require a clean install. See how its done:
https://www.groovypost.com/howto/downgrade-windows-10-to-windows-7-windows-81/
I also suggest that it will be best to reach out to the VLSC Team with regards to this matter so that they can provide you with the exact information that is needed before you decide to make a purchase: https://learn.microsoft.com/en-us/licensing/contact-us
Best Regards,
Hania Lian