The Gateway probably wins, hands-down, for combination of oldest and "netbook"; although it wasn't really possible to use it as a "netbook" when it was new.
The roughly same age, very-much-'net' device that would probably be closest to "first netbook" is the HP OmniGo 700LX. Essentially an HP 200LX, MS-DOS-running "palmtop" computer with a cell phone modem built-in. Always-on 1G data access. Complete with 'dock' for a Nokia cell phone.
My first system that I would consider a "modern netbook" in the current usage is my old Sony PictureBook. I bought one in late 1999, and had a Lucent WaveLAN card in it by early 2000. (I also worked for a company at the time that provided the first "public hotspot" in Portland, Oregon, covering Portland's Pioneer Courthouse Square.) Roughly equivalent to the Toshiba Libretto, it added a wide aspect-ratio screen and webcam, just like most modern netbooks now have.
The roughly same age, very-much-'net' device that would probably be closest to "first netbook" is the HP OmniGo 700LX. Essentially an HP 200LX, MS-DOS-running "palmtop" computer with a cell phone modem built-in. Always-on 1G data access. Complete with 'dock' for a Nokia cell phone.
My first system that I would consider a "modern netbook" in the current usage is my old Sony PictureBook. I bought one in late 1999, and had a Lucent WaveLAN card in it by early 2000. (I also worked for a company at the time that provided the first "public hotspot" in Portland, Oregon, covering Portland's Pioneer Courthouse Square.) Roughly equivalent to the Toshiba Libretto, it added a wide aspect-ratio screen and webcam, just like most modern netbooks now have.