Hi, I managed to solve this by copying the texture node and change its vector input outside of the group, but I wonder if there is anyway to do this inside once the image is changed to a color node? Since the texture coordinates are only available on the Image Texture node from the start and I want to change the UV coordinates only from the green channel, thanks.
obviously this doesn't work
Welcome to BA 
You can drag any socket to the Group Input node and it will show up as a group parameter