shining force question

Unless you've sold it, you've got the gravity stone by now.

IIRC, Galm gives it to you.

Have you got to the spiral yet?
IIRC, you cant sell any items that affect the story line. So it is possible that it is your inventory from your reserve characters, or haven't got it yet
You can sell the gravity stone, but you can buy it back from any tool shop. You know, the one that sells herbs...

If you are actually up to the South Shrine then you would have the gravity stone by now, and IIRC you don't need to 'use' it, you just have to have it, so it shouldn't matter who carries it.