Can you get Gmail in other browsers on that computer? If so, look at:
about:config -> noscript.ABE.localExtras (if not default, then reset)
*and*
noscript options -> advanced -> ABE: if "WAN IP(...) E LOCAL" is checked, make sure it's showing the correct WAN IP
about:config -> noscript.ABE.localExtras (if not default, then reset)
*and*
noscript options -> advanced -> ABE: if "WAN IP(...) E LOCAL" is checked, make sure it's showing the correct WAN IP