In the key of G the basic chords you will use are G, C, and D.
In the key of D the basic chords you will use are D, G, and A.
In the key of C the basic chords you will use are C, F, and G.
These are the I, IV, and V chords of each of these keys (the most often used)
You can play out of the keys of C and D without using your capo, just by playing the chord patterns that go to that key. If you know how to make each of these chords.
Hope this helps, if not ask and I’ll try and explain further or might write the chords out in tab. Is there a specific song you are trying to do in the key of D? This might help the decision of how to approach it as all 3 different ways to play out of D that I’ve mentioned have somewhat different sounds and lend themselves to different songs better or worse.