IT WILL SAVE YOU MONEY. It Will Perform 10x Better Than A Gateway.
Tell them that ^
Given that a prebuilt pc you buy at WalMart will cost $400-500, it will be a complete peice of garbage compared to building your own pc for $600.
The benefits really are common sense when it comes to building your own pc, a $1000 computer that you buy from Origin/Digital Storm will have lower performance than a $1000 computer you picked out yourself. Almost always.
It's so much easier building a pc than you think, sites like Toms, pcpartpicker.com, will help you out with compatability and performance per dollar.
Offer to put in a few bucks of your own if you want to spend more than they are willing to, so you can build something that is worth the money.