I have never fished Gabriola, so this is an educated guess that you should take with a grain, or two, of salt.
The Ambleside fishery is a a terminal fishery; you are primarily angling for fish that are staging in the saltwater waiting for the opportunity to migrate upriver. Gabriola doesn't have that. The salmon fishery on eastern shore of the island in Gerogia Strait is for migrating salmon largely bound for the Fraser and Columbia. The fishery is in deep water offshore. I would suspect very little opportunity for shorefishing for salmon on the eastern side of the island.
There are coho that rear in Georgia Strait. You might find them foraging in/around kelp beds near the shore.
For shore, or kayak, fishing I would suggest the northwestern end of the island. You may pick some sea run cutthroat that are native to the Nanaimo River system, or some staging salmon for the same system. Also the south end of the island looks fishy for shore fishing as salmon will pass closer to shore as they navigate the pass.
Be aware of closures and RCA in the area.
Seeing as you have not gotten other replies, I thought I would throw this out as an educated guess and a Hail Mary. If you get better first hand info that contradicts what I said, go with that instead!
Cheers.