I'm assuming both computers don't have the same OS?? If they don't all you can really do is just transfer files such as your my documents type of stuff, just don't transfer programs or system files. If it were me, I wouldn't try networking... but if you are going to need your computer networked you might do it that way.
The easiest way to transfer files is to just do it directly in your computer. (at least it is for me, maybe you won't want to) Just put your gateway HD in the dell as a slave. (if you use cable select put the gateway HD on the gray position of the IDE cable... [drive 1] --middle... otherwise jumper the gateway as a slave) Check the bios (most likely [del] to get in) to make sure both HD are recognized. Load windows and you can access both HD from my computer and you can transfer what you need from there.
Maybe someone will jump in with an easier way, but that is how I do it. Let me know it it works ok. Some times there are things that cause a HD to not be recognized but it should work ok.