I had the same problem and found the solution here: https://www.reddit.com/r/feedthebeast/comments/4zp2ku/is_it_possible_to_add_mods_to_the_1102_forge_dev/ (by brute force googling)
Go to Run/"Run Configurations ...", select in tree left "Java Application"/<Name-of-your-ProjectFolder>_Client, go to "Arguments" Tab and remove in "VM arguments" "-DFORGE_FORCE_FRAME_RECALC=true" what should be the whole content of that textfield.
This stops FML/Gradle to use any mod in its deobfuscated version; what means methods names, variables etc. in the foreign mod jar file don't get renamed into a human readable version while loading.
With deobfuscation the mod become unable to load parts of itself by reference the name of classes/modules. If you want to add references to other mods (e.g. for dependencies) in your own mod you might need a more elaborated method.
I would love to see the forge documentation extended by a section that explains deobfuscation much better than I try to do in the lines above ...
