1811GNR's info on the G-series is correct as far as it goes, however ALL Browning imported FALs are actualy "G" series. It's just that after 1963, they didn't have the sear cut. I owned one.
Most people think that the "G" series guns are all sear cut ones and that's not true. There were many more "G" series guns imported after the ATF ruling that grandfathered the sear cut guns. The later "G"s aren't sear cut and are virtually the same as any other civie FN-FAL import and are perfectly legal.
Some of these later "G" series guns get passed off to the unsuspecting as part of the much more valuable grandfathered sear cut guns.
The "G" series will have a "G" prefix on the serial number. You then have to check if it has a sear cut or not. If it doesn't, then it's a later Browning import and while valuable and a great rifle, not qutie worth what most people think of as a "G series". If it does have the sear cut, you then MUST check the serial number against the list of grandfathered serial numbers that the ATF considers legal. If the gun's serial number appears on the list, then it's the real McCoy and worth a ton of dough. It the gun's serial number does not appear on the list, it's an illegal machine gun. There is no in between on that.
So make sure it's a "G"
Make sure it's sear cut
Make sure it's legal
Those are the three things you have to do to insure it's one of the famous, high-value guns. Remember that there are plenty of non-sear cut "G" series guns out there that are not part of that group. The non-sear cut gun I owned, and the couple others I've seen were all serial numbered well above the group of sear cut guns. They all had an extra digit in the serial number from those that were the first, sear-cut guns.
Value wise, those sear-cut "G" guns will increas in value, but really if you're looking at investment and can legally do it, a class three FAL might be a better investment. It seems as though value on those increases greater, and if you get a real Belgian FN gun there really isn't anyone that can say shit about you not having a "real FAL". You would also be able to shoot and enjoy the gun, and otherwise use it while it's value contiues to skyrocket. The "G" guns are far more collector pieces and using one would probably effect value to a greater degree.