What I tend to do, is visit the Gateway support website and locate that model# and review the "support" available. The manual shoudl explain what HD is required or the specs as it was build. As Robert offered, if its that old, you need to understand what you're dealing with. If as he suggests the HD is EIDE(PATA) based, then get another good one. Your OS also, probable XP??? that should installl on the HD but there is a 127Gb limit on older PCs, that is overcomed by either using HD vendor install s/w or using at least XP-SP2 or better to install larger than 120Gb drives. The bios too has to set to acknowledge, "LBA" for large HDs. All this is explained via googling it all or similar.
Alas, if you plan Vista ort better OS, then that's a moot point. I suggest then that the desktop be ready and proceed foward. Std. HD for a E6100 it seems was 80gb, not much. Install all the ram it can handle and once OS is installed, verify the current bios version. I tend to update the bios regardless, to the best latest in order that it have all it can be. If you like, then check into a separate SATA capable PCI-based card to upgrade abit if you want SATA capability, just be ware of the limitations of that card and what it requires. If you remain at EIDE, then 500Gb is probable as high you go for costs but better is available. in other words, you have a limited PC due to age and generally speaking more added cost unless you have access to used or 2nd hand items/devices, etc. to improve your lot. enjoy...
tada -----Willy 