All nominally 5/16th inch:
Not all 5/16 inch chains are the same!
Peerless ACCO G43 has a inside link length of 1.03 inch x 0.5 inch wide and a wire diam of 0.329 inch
Peerrless ACCO G30 has an inside link length of 1.1 inch x 0.507 inch and wire diam of 0.342
Peerless ACCO BBB has a link length of 1.000 inch x .5 inch wide and wire diam of 0.328 inch
Peerless Boatman's Pride G30 has a link length of 1.1 inch x 0.507 inch wide and wire diam of 0.342
Peerless Boatman's Pride G43 has a link length of 1.03 inch x 0.49 wide and a wire diam of 0.33 inch
Campbell G3 has an inside link length of 1.1 inch x 0.5 inch and a wire diam of 0.34 inch
If your chain is domestic is should be one of these, if its an import
it might be similarly sized.
With a pair of calipers and a micrometer you should be able to sort out your chain type. But you need take a number of measurements as the gal might have different thicknesses.
There is a considerable difference in these link lengths. I do not know your gypsy but vertical ones are more forgiving as they sometimes only 'take' 3 links (or 90 degrees) horizontal gypsies take lots of links as the chain wraps round about 270 degrees and any difference between gypsy and chain compatibility causes the chain to jamb.
Not being an American it seems odd you can have a gypsy simply marked as the 'nominal' 5/16 inch when there are a number of variations of 5/15th inch?!
The only, or safest, way to get the correct chain is to take the gypsy off and size it with the chain in the shop/chandler. Gypsies are inordinately expensive, almost as much as new chain (or they are in Oz!)