Stimson has just first-choice pairing Garry Richards and Simon King at his disposal ahead of the club's return to League One, although youngster Tom Wynter and midfielder Mark Bentley could fill in if required.
The Gills had former Yeovil man Aaron Brown and ex-Crewe centre-back Chris McCready on trial at the start of pre-season training but Stimson allowed them to leave and admits it is proving difficult to find players at the level he wants.
He said: "There aren't many good centre-halves out there if I'm honest. It may be we have to go down the loan route with a younger player like we did last year with Leigh Mills. There are a couple of people out there we're looking at but again it's down to finances."
Meanwhile, striker Simeon Jackson will be heading back to join his team-mates for the rest of pre-season in the near future after his Canada side were beaten 1-0 by Honduras in their CONCACAF Gold Cup quarter-final on Saturday night.
Jackson, 22, came off the bench 14 minutes from time to win his sixth international cap and came closest to levelling for his country when he headed against the crossbar, but they were unable to find a leveller and were beaten by Walter Martinez's first-half penalty.