I'm not a lawyer, but I've been using the rule, if the modifications are in the qagame, cgame or ui library files, they must use the same license as the original source. With that, if you use any code base that originates from the ET SDK, it should be licensed by the ET SDK license. If your code base starts from the ET GPL release, then it must always use GPL 3 or compatible license. For a fact you can use the ETPub source and modify it. The limitation is that you can't use it to make money in any way. As is prohibited by the ET SDK license. I'm not really sure if the ET SDK license allows the creation of a completely different game from the ET SDK source. But I don't know any case where it would have caused anyone problems. If someone else makes the same exact code as you, you both own your own separate codes. It doesn't matter if they are exactly the same.