Hi OP, I had an ECG last Christmas and was really worried about the same thing. I have PTSD related to medical procedures and I struggle with taking a sock off let alone my top and bra. Everyone I spoke to said "you have to do it, it's only a minute, it's nothing" but to me it isn't "nothing" it's very much everything, even after therapy.
Mine was at the hospital but it would've been the same at the GP's. I told them before hand that I have a problem, I took my partner with me for support and I made sure the person doing the ECG knew that I wasn't comfortable, my partner did this for me before I went in.
I did have to take my too and bra off, and I did do a bit of a cry while it was just me and my partner behind the curtain but I was given a big towel/blanket to cover up with and I was lying down and covered before the technician came in.
You have to move around a little, leaning to the side and turning a bit but I was covered the whole time. Her hand was under the blanket with the probe but she never actually touched me, it was only the probe.
She left us alone for a minute afterwards so I could get sorted etc and was really accommodating.
I don't know if that helps at all, but if you explain before the procedure that you don't want to be exposed and if you're not covered then you don't feel comfortable continuing then you should be ok. Take someone with you for support and make sure they know how to advocate for you if needed.