You can fish all around GKI except for two places that are designated green zones. These no-fish zones are located on the western side of the southeast peninsular and extend to the west to be inline (looking due south) to the northern tip of Halfway Island. The second no-fish zone is north from Monkey Point and includes the entire area of the small cove . Above that (Fishermans Beach?) it is a yellow zone. Grab a map of the green zones put out by the Qld Gov. This will show these areas in detail. The map you need is MPZ17, The Gladstone area.
The headland called Big Peninsula is a good place to try for mackeral and other pelagics including tuna and cobia. There should be some great whiting around off the beaches and also some flathead. I dont know about boat hire either from the resort or the mainland. There are fishing charters that you can go on. The coral reefs that are found around GKI are wonderful to snorkel on. Most of these can be easily reached by swimming at low tide. There are many beautiful sea creatures there and many of these have never been identified or named. PM me for more info if you wish.
Eagle