The SourceMod License is printed below. There is a short FAQ as well.

Click here for full text.

Q: Is SourceMod open source? Yes.

Q: What license do plugins/extensions fall under? Derivative works must be under the GPL, version 3. There are compatibility expections for using 2, and these exceptions are listed in the full text linked above.

Q: Can I sell/charge for plugins or extensions? Sure, but you have to make the source code freely available as per the GPL.

Q: If I give someone a plugin or extension, do I have to make the source code available? Yes.