Just for the curious there are...
8.0828e+38 possible game card codes.
That is the following number for those not versed in scientific notation.
808,280,000,000,000,000,000,000,000,000,000,000,000 possible game cards. (Actually it's a bit more, this is rounded off at the last

.
That's 808.28 undecillion possible game cards.
I doubt there are more than a million -active- game cards.
So that gives you the following chance to -guess- one.
1 in 808,280,000,000,000,000,000,000,000,000,000
That's One in a 808.28 Nonillion chance.
For those curious on the math of this, 36 possible characters filling 25 slots.
That big 808 number is 36^25.