I read somewhere that Avatar had the advantage of exchange rates that were 10-15% better. That’s probably the main reason. At least TFA beat it in the US.
Regarding Gone With the Wind’s all time adjusted record, it will never be beat. Simply put, for 40+ years following Gone With the Wind’s initial release there was no such thing as home video, so the only way for people to see it was in theaters. Indeed, GWTW made most of its money from rereleases. That doesn’t even get into the fact of modern internet piracy and the fact that in a few months TFA will be on Blu-Ray, DVD, iTunes, Amazon, Youtube, HBO, torrent sites, etc.
But TFA’s run will be impressive when adjusted for inflation. In fact, it already is. Right now it’s sitting at 15th place all time adjusted, ahead of every Star Wars film save the first two (though it will overtake ESB in a few days). When all is said and done, it has a chance to make it into the top 10, which is VERY impressive.