Quote: "so does this mean that when we get the release version 2.0 does that mean i can charge money for my projects too ?"
If you have a commercial license serial code in your DarkGDK.com account then it means that you can sell the games you create.
Quote: "but why is there an expiration date to this?"
Because it is just a release candidate (i.e. it's not a final release) and it's available for free to anyone until it expires. Now when it expired only those who upgraded from GDK 1.0 commercial or bought a separate GDK 2.0 commercial license will be able to compile.
Quote: "I bought DarkGDK 2.0 but when i downloaded RC4 it says that it is expired"
As I said, if you paid for GDK 2.0 commercial you should have two separate downloads - one BETA and one COMMERCIAL. Download the commercial one, register it with the commercial license key and it will not expire.
Intel Core2Duo 2.2GHZ, 2GB, GeForce 8600M GT 1280MB, Windows Vista Ultimate SP2, PureBasic 4.61 + DarkGDK 2.0