What type of map are you looking for? There is a map chip for Ontario that has GullRock lake, it’s …okay. It will get you close to some of the major reefs/underwater structure but it’s not exact.
A lot of the resorts will have a map for you once you get up there, I think they may all use the same one.
Otherwise this one is a pretty good basic map:
http://www.poplarpoint.com/wp-content/uploads/2015/12/Gullrock_lakemap_poplarpoint1.jpg
When are you going up there and where are you staying? I’ll be at Five Lakes Lodge at the end of May and over the 4th of July. It’s an awesome lake, and Five Lakes Lodge is a top notch resort(no experience with the other camps).