harddrives are completely independent of the computer. there is no "matching". your gateway probably has about 20gb of storage. i would suggest at least 60-80. i have 120 and its holding me very well. best drives are seagates for quiet operation and western digital for all out performance.
as far as a cd burner forget all other brands. LiteOn is the brand you want. Its the best drive available short of paying twice as much for a Plextor. LiteOn makes all Sony drives, Memorex, TDK, and many many more companies. They basically own the CDRW market. The prices are very nice as well. you could get a 48-52x (top of the line) for around 55 bucks at
www.newegg.com
you can't go wrong with the LiteOn's. I have a 12x LiteOn that hasn't failed me yet. I plan on upgrading to a 52x LiteOn soon. i wouldn't even give a second thought to any other brand.