Welcome to TSF! According to this document:
http://support.gateway.com/s/Mobile/...shtml#P25_1569
The system doesn't have a modem or ethernet jack/support. You'll need to utilize a PC card for any of those functions. I suggest getting a Xircom, since they're all in one...
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Repair Windows XP Guide ---
Resolutions
Crazy Computer that I'm saving up for::Antec P180 Case;PC Pwr&Cooling 1kW Quad SLI PSU;Asus P5N32-SLI SE Deluxe;Intel Core2 XE X6800;Corsair TWIN2XP2048 2GB PC2-6400 DDR2 RAM;2xAsus GeForce 7950 GX2;BFG PhysX Accel.;IBM DVD-RAM/Plextor DVD±RW;Seagate 750GB HD 7200RPM;WDX 150GB 10000RPM;Vigor Monsoon TEC Cooler