Guitar volume pot quieter at 10 than at 5

The one time I experienced that (hum increasing as the volume pot is rolled down) the problem was a cheap cable with poor shielding.

 

You might first try cleaning the pot with some DeOxit or similar -- just to eliminate it as a cause.
If that doesn't work, then escalate up to solder joints and/or swapping the pot out.
Also, you might have a look at the wires/connections at the output jack.

 

That sounds similar to a problem I had once with a bad ground on the vol pot. You'd think it wouldn't work at all, but it acted like that instead...

However, if you're fishing pots through F holes, I'd probably just replace it. No fun doing that more often than necessary.

 

If blasting it with a plastic safe/no residue electronics cleaner doesn't fix the issue, replace it. Pots are cheap. The ones that came as stock equipment in that Gretsch are even cheaper

"Plastic safe/no residue" is super important if you're shooting it into a pot that's mounted in the guitar because the spray will get on the wood and any lube/solvent can stain the wood or damage the finish through the wood.

If you decide to rewire, remove the pickups because most of the work can be done through the pickup cutouts. Use string or plastic tubing to pull the pots into place. Take care not to drag the wire loom and components on the guitar's top because they will scratch up the finish.

 

Glad you sorted it. However, I don't understand how a poor string ground to the bridge can cause that symptom.

Was it due to a bare wire (the string ground) in contact with tab 3 of the volume pot?