Do I need to create a new course with just a front nine and another for the back nine so when I play those it will accumulate the stats?
If you enter 9 holes (or any number of holes less than 18) for a “round” and get results, everything is scaled to 18 holes. If you get results for, say, one round with 9 holes and another round with 18 holes, then the results are computed for all 27 holes, then scaled to 18 to give results on an 18-hole per-round basis.
And you can record a ‘round’ with any number of holes. For example, you can record holes 3, 7, 9, 10, 13, and 15 and the GM app will give you a report on that ‘round’ (i.e., with results scaled to an 18-hole equivalent round). If you ask for a report on multiple rounds that includes, say, 41 holes, all holes are included (equally weighted) and then results are compiled and reported on an 18-hole basis.