Alot of people think the G test is all about highway but its actually everything.
The main key is that the G test is
All About Observation. So the instructor wants to see you scanning at intersections, checking blind spots, Rear view mirror when braking, signalling properly in three point turns. The highway portion is the easiest since you merge in, they ask you to do like 6 lane changes and then you exit.
What helped me was doing the same thing I did for the G2, Say aloud, what your doing. Example: when merging, say under your breath "checking blind spots".
These tests can be nerve racking cause its a pain to reschedule and do it again but don't worry about it, its nothing like exams