Your counselor is right about there being limitations on the courses you can take using the GI Bill, but it’s not as simple as all being in the same major. In order for the GI Bill (regardless of whether it is the older Montgomery GI Bill or the new Post 9/11 GI Bill) to be used to cover school, you have to be enrolled in an an approved academic program, which is generally a degree program. It can be any degree, but all of your courses have to count toward completion of the program in order for them to count toward eligibility for the GI Bill.
So, while you can’t take random courses, you can enroll in your choice of a degree program. You’ll have to work out with your academic counselor which program best fits your needs. You should note that it is possible to take courses toward a second bachelor’s degree, for example.