As to Java version, what version are you running on? I have MC 1.7.10 through 1.10.2 installed, and GC and other mods too, they all use Java 8 JRE 1.8.0_121 (January 17, 2017).
Took me at least 3 hours to find a way to make it work, and now it's fine. I've encountered countless problems, having to solve each in unique ways. I had a great time fixing everything, I may do a post on how to fix all those problems. I'm totally happy for now. Glad I found that Forge patch that makes things work with Java8. Also, btw: the simple way is to use 1.7.10 just as you do, should have no problems. I wanted 1.7.2, where, well... sh|t happens.