I'm not sure of your relative skill level, so I'm just going to explain what I'd do, if you need step by step instructions let me know though.
First I'd figure out what my JAVA_HOME environment variable is, and which javac binary is being used. Then I'd backup whatever javac binary is being pointed too, and link to the gcj javac instead. You might have to fool around with JAVA_HOME, but ideally you could just run the gcj javac and be good to go.
As I said, ask for specifics if you need.