As for the graphics card comparison, the 8700m GT gets maybe 4600 points on 3DMark06, while the 8800m GTS gets at least 7100 points on the same program. The 8800m series is much more powerful than the 8700m series.
The sound...well the Toshiba advertises having 4 speakers and a subwoofer built in, while the Gateway doesn't say anything about it, other than saying 2 channel. That seems to imply that there are 2 speakers built into the laptop. So if you're going for straight sound quality out of the laptop (which, no matter what, won't compare to discrete speakers or a good set of headphones), get the Toshiba. Personally, I would still get the Gateway, because it's easy to buy a set of speakers and hook it up. It's hard/impossible to upgrade your graphics card.