Well, it wouldn't be G, because it's a mite difficult to "capo down"! You, as the songwriter, don't have to "do" anything -- it's up to the guitarist. Some will just play it in F (and my hat's off to them), some will capo up one and play in E, and the duffers like myself will probably capo up 3 and play it in D, sacrificing some of the sound quality for easier chord forms. If you're a better singer than your accompanist is a guitarist, you may want to do your song in G, though I understand that a particular key may suit your needs as a songwriter better than another. But I'm an amateur, so I'll gladly defer to those with more knowledge and experience.

Aloha,
Mark