Well the best advice is to really ask your gf about
it.
Have been mingling with lots of Christians lately (and also
attended their bible studies), and I can tell you that their number
1 rule with regards to marriage is cannot marry
non-Christians.
Yes, they can date non-Christians, but under the eyes of their
god, a true marriage is when they are both Christians. But if your
gf is the super chinchai kind, always skip church and not very
involved in her religion, then you probably have chance of marrying
her.
I think one reason they have to marry Christians is because they
believe that when you both die, Christians will go to heaven to
spend the eternity with god, while non-Christians will be sent to
hell to be separated from god forever. So if you are not christian,
you and your girlfriend will be separated forever.
Also, it's not a good idea to pretend you believe in
christianity just to marry her. Because if you do, 1) your
relationship wil be built on a lie, 2) you will be miserable
attending church/bible study/cellgroup or whatever thing
weekly.