by the time I made it to a GI I already had a dx and tissue biopsy so I didn't need to do much.
but when I initially discussed by symptoms late last year with my PCP, I came in with a list of all symptoms and date they appeared. I was exhaustive with my list (I put canker sores on there even though I haven't had one in a couple of years, for example). I also had bullet points with questions that I made sure we got through--this prevents me from getting side tracked. I think this worked for me because she referred me to a GI who booked me for an upper endoscopy and colonoscopy (I just never made it to the appointment because I ended up in the hospital first.)
good luck - I hope you're able to get some answers!
I always bring someone with me - either my FI or my mom. I tend to get incredibly nervous as soon as I walk into the room. They don't even bother to take my blood pressure anymore because they know it will be high just because I'm there. If someone is with me, I know they can ask questions I forget, and they'll better remember what the doc says.