Jez,
not flown in QLD, but familiar with it. Gympie itself is a regional town, not a lot happens there, but as with anywhere in Oz, good tucker to be had. Not a place to go out of your way to see. No idea what the airfield facilities are.
Fraser Island on the other hand ...... unique. There's no strip as such at Eurong - the beach is it! The beach runs up the east coast for about 60 miles. Eurong is just off the beach, and should be easily identifiable from the air. Best to fly in at low tide - the high side of the beach is pretty soft sand, the bit that is covered by tides is very firm. It tends to have stable offshore sea breezes (i.e. cross-wind). Getting off the beach to Eurong is a short walk up a 'road' through steep soft sand. Thing to watch out for is that the beach is the main road for the island - so you get all sorts of 4WD vehicles driving up and down it. Easy to spot, though. Be aware that the RAAF often uses the coast as a route for F111 training - I've seen them off Fraser and also further down the coast near Maroochydore. They fly fast (500+mph I'd estimate ) and low (100-200 feet) and pretty close to shore. Not nice to get in their way.....
Well worth a visit..... See if you can get a lift up the coast about 10 or 15 miles to to Eli Creek for a swim - it's heaven on earth. Freshwater, crystal clear, fast running through the jungle - just sit in it and enjoy the ride.....you may also be able to land on the beach at that point, though I'm not sure.