You can use the C64 executable produced by the GCK even for commercial release without problems or restriction (if you will use the given assets of LSS2 inserted as example into GCK for commercial release, you can use the the provided charset and sprites, however musics, levels and pics should be changed as created specifically for LSS2 to avoid confusion).
If you are a programmer you can also modify the ASM source of the game inside the Java program if you need to customize the engine and the C64 executable produced by it will be as stated above too.
You can modify the Java source code of GCK but redistribution of the Java source code and Java compiled JAR should be authorized. The C64 executable produced by modified Java source code can be used for commercial release as stated above without problem.
