This can only be done by editing the source code, and of course, you cannot remove credits, as Glest's license requires they be credited. However, I would recommend you do NOT do this, as then you would have to recode that part with every new release of GAE in order to keep GAE up-to-date. What I recommend is to simply add your name in some parts of the media or include a credits file, as most people do with their mods. For example, you can edit the logo so that it has the name of your mod and your name. Note that the latter is the least obtrustive method.