I had reason to believe i had an autoimmune disease but my gp disagreed. i decided to go get tested anyways (i was right) how do i approach this?

Gingerly: Tell your GP that you got tested on the advice of a friend (maybe us) and share the results. Say you were surprised, but trust that s/he will help you understand and deal with them.

You can: Still request your GP to some simple blood work up or you can go to an Immunologist

Talk to her GP: You can return to see your doctor to tell him of your diagnosis. If there is a diagnosis. The specialist can do that too. Depending on how he/she reacts and if you feel uncomfortable, you may want to see another primary care doctor. But at least talk to or even write a note. As physicians we are in a constant state of learning. We can all make mistakes.