Yes, that happens with DIY downgrading.
https://support.gateway.com/us/en/emac/product/default.aspx?modelId=1578 has a link to detect the exact hardware you have. And it tells it's an Intel driver. So now, it's up to you, armed with the knowledge from that tool, to find the right driver on the intel site.
Same, of course for the other drivers from Intel, and the Sigmatel and Conexant drivers, if those aren't on your Windows XP disk.
But it might be easier to just do a system recovery back to factory conditions to make this PC just as fast as it was.
i have a emachines t3604. it was originally vista basic but i brought it down to xp pro to run faster and now i need the video controller (VGA compatible) and i can not find one anywhere. does anyone have any ideas?