I would like to bump this topic as I encountered the same issue, yet ForgeGradle 2.1 is already there.
I'm using build.gradle from latest git branch (https://github.com/MinecraftForge/MinecraftForge/blob/1.8.9/build.gradle)
If I add entries from the MDK, as you suggested, I have some conflict with apply plugin: "net.minecraftforge.gradle.patcher" line, then if I comment it out, it complains about version format (1.8.9 seems to be too short) but I guess such try&test leads to nowhere...