Do you have a picture of the existing platform and transom ladder arrangement?
On my boat (trunk cabin design like a GB, but a little higher freeboard) I've got the arrangement pictured below. The swim ladder is hinged and flips up against the transom ladder for storage (it's up in the picture). I do wish the swim ladder had a 4th step though, as that would make usage easier.
For hand holds when climbing up, I generally grab the platform slots at first, then once I've got myself up a bit onto the ladder, I use the upper ladder as a hand hold while climbing the lower ladder. Once on the upper ladder, the side rails of the ladder (and stern rail) provide hand holds.
For perspective on sizing in the picture, with full tanks the platform is about 6 inches off the water. Height from platform to aft deck is about 3.5 feet. So the ladder only extends into the water about 2.5 feet, which is a little short. A telescoping extra step would fix that without affecting the ability to use the upper ladder with the lower one flipped up.