So Im using Gnome and Im trying to share a folder
the method i followed is to go to the file manager (nautilus)
and right click and select “Local Network Share”
but that option is missing
and a solution i found online is to install nautilus-share
but that package doesnt exist in nix packages 
Any help would be appeciated
It would need to be packaged.
The original project was long unmaintained and incompatible with modern Nautillus but looks like one of Nautilus maintainers tried to resurrect it: Corey Berla / nautilus-share · GitLab
You can try nix-init and then repeatedly building it adding remaining dependencies. You can also look at nautilus-python for inspiration.