I am unable to run the gradle task runClient in the 1.7.10 branch. The task runs and minecraft starts its initialization progess but then a window opens which title states:

Select an mcp conf dir for the deobfuscator

It is by default in the eclipse directory of the branch. When I select this directory I get an error and the same window opens again. After the third time of selecting the eclipse directory the task terminates. Here's the log of the task.
task_output.txt

Before running this task I successfully created the development environment by running setupDecompWorkspace.

So how can I get this to work?

Fixed it by pointing it to a directory named $USER_HOME/.gradle/caches/minecraft/net/minecraftforge/forge/1.7.10-10.13.4.1566-1.7.10/unpacked/conf/ where the needed files are present.

This is actually an issue with ForgeGradle, and not our fault. So since you managed to resolve it, I'm closing the issue.

Actually that's CodeChickenCore looking for mappings for it's runtime deobfuscation.

Well all I know is it ain't my problem.