I had been driving high way for 1 year before I took G test. Before test, I read some postings in this board, and chose test time 1:25pm when there was no much traffic. Note: check mirrors often, check every intersection. On the test day, before the officer came in my car, I turned on my AC, and turned on light music on low volumn, just made the car comfortable for both of us. That sure did work. We both were quiet relaxed. And he even told me what the speed the road was. On the way back, he didn't ask me to drive on highway. Anyway, my personal opinion: No need to have an instructor for G test. Read postings on this board, check often. Relax, be confident, it shouldn't be that hard.
Good luck.