Now that 3D Realms are no more will their games continiue to be offered/supported on GOG?

Yes.
The rights are held by the new Apogee Software, not by 3D Realms. I believe GoG's deal is with Apogee.

While 3D Realms is no more, the recently relaunched Apogee is very much alive and well; they are currently developing a portable title called Duke Nukem Trilogy. While legally speaking Apogee is a separate entity, in practise they have strong ties with 3D Realms and will no doubt continue to maintain their collective IPs as if nothing had happened.

Thats a relief... now I can relax... thank you!

The deal with Apogee it's also in their new website. :)

good to know
I was wondering whether or not my cash was actually going to 3D Realms/Apogee or not. Good to confirm it is.
Glad I bought it now.