I've just found a proxy somewhere and applied to my code, seem to be successful. But when it comes down to submit("btnG"), I got an syntax error SyntaxError:
syntax error (httpunit; line 155)
at org.mozilla.javascript.NativeGlobal.constructError(NativeGlobal.java:597)
at org.mozilla.javascript.TokenStream.reportSyntaxError(TokenStream.java:1324)...
Submit from net.sourceforge.jwebunit.junit.WebTestCase is deprecated, so if this is what you are trying to use then you will have some issues