To be honest, it's probably more beneficial to move onto the further content/textbook as it won't be too hard to pick up.
Although I'm guessing you don't have the further textbook yet and so want to do stuff out of the GMA book.
Whats Basically relevant, will depend on which modules you end up doing in further. You'll have to do the core module which is stats and such, but out of the rest you'll only choose three.
I'm going to pick the chapters out of the Essentials book because that's what I have sitting here to go off of, but it should be similar for other textbooks.
Ch 1 - Matricies will be handy if you do the matricies module
Ch 5 - Sequences and series will be good for the sequences module.
CH 9 - Ratios and Similarity for part of the trig module.
Ch 12 - Trig ratios and applications again for the trig module.
Ch22-23 covers part of the stats module.
But seriously, if you can get your hands on the further textbook, it'll be better and save you a bit of time as Further isn't a unit that you'd struggle with without the chapters from GMA (and well you can do Further without the 1/2 units and still do well).