welcome to Male Survivor!
Be aware that you are not alone , I'm sad to know about your abuse but healing is possible even many years later every one of us is sort of evidence for that. I found great support here!
I would suggest you not to give some details in that talk with which you are not comfortable or that could be emotionally too exhausting for you. Saying that you were abused and that you have later problems should be more than enough.
When I visited my therapist first time I didn't know what to expect and I was scared and worrying. When I meet her I told her in short all my story. Later she told me that was not necessary, trust and supportive relationship between therapist and client is something that is gradually building so there is no need to force anything. For start just couple of facts should be more than enough. Same goes for your talk with GP.
So please don't push yourself!