They are probably on the lower end...I would look into lifestyle changes that you could make in order to raise them anturally...also supplements such a tribulus and Alpha Male seem to work pretty well for guys in your situation. Controlling estrogen also has a positive impact on T levels (Rez-V, losing fat, etc.)
Since you seemed to react positively to the exogenous T, this could mean you are meant to have higher levels. See what you can accomplish naturally first--you will have about a 0% chance of finding a doctor that will put an 18 year old dude on TRT with T levels in the 500's.
Are you saying you got stretch marks on your armpits and groin after one week on the gel? I find that rather hard to believe.