In reference to what Hopalong said about installing XP, if the option is available from the bios (for me it is F8 while on the bios loading screen) to pop up a boot device menu the easiest way to dual boot is to,
1. Disconnect the drive with Windows 7 on it
2. Connect the drive for XP and install
3. Once XP is installed and working then reconnect the Windows 7 drive
4. Check the bios, manual or even possibly it will show up on the Gateway loading screen (what you probably have) and see if it says which key to hit. If it is available after you hit the correct key you will have something that looks like this,
where you just select which hard drive to boot from. Even if that option isn't available it would still be easier to to the above and then enter the bios and change the boot order to the drive/OS you want to load than having the Windows bootloader.
As far as drivers for it goes, assuming the specs for yours is the same as what is here (780G chipset with teh HD 3200 integrated video), Gateway Official Site: Shop - Desktops - DX4300 15e Desktop Product Details
then XP drivers are not a problem despite what Gateway says, just get them straight from AMD, ATI Radeon
If you need the AHCI driver to get XP to install (the F6 step at the beginning) grab this, XP.zip
That is the XP AHCI/RAID/SATA driver (was easier than posting directions on how to get it).
for the audio you can get that straight from Realtek, Realtek
Not sure what it has for the onboard NIC (Gateway only shows a Marvell-Yukon driver for all OS'es, and is dated from 2005 ????), just check what it shows in Device Manager in 7 (same can be done with the TV tuner shown in the specs if you plan on using it in XP).