When out of tonic, one must resort to other options. We do have some Sprite sitting around.
Lemon-lime soda serves perfectly well. It covers the gin taste nicely. Perhaps it is too sweet. The bitter lemon from a couple months ago had more dimension - more natural flavor, maybe? Tonic is still the go-to for me. It's readily available at any grocery store and it's certainly cheap. But Sprite made for a nice change of pace.
Perhaps a Sprite/7-Up blind taste test sometime? That could be interesting. I had a friend growing up who swore he could taste the "lymon" in Sprite and thus preferred it. Ah, the power of marketing... Or is that the power of lab-produced flavoring?