This problem occurs on certain computers that don't have enough RAM available to dedicate to the minecraft install. Thankfully, there's a fix for it!
First off, you'll need to open up the Command Prompt, you're going to have to run the install manually instead of using the executable.
Once you have the Command Prompt open, you'll enter "cd " without the quotes, followed by dragging the Forge folder on top of the Commands Prompt window. Hit enter and you'll navigate in to the Forge folder.
Now you'll have to enter two different commands. The first is this:
gradlew.bat -Dorg.gradle.jvmargs=-Xmx4G setupDecompWorkspace
I'd recommend entering that entirely by hand, copy and pasting can sometimes cause problems. Also make sure your capitals and spaces match the command I provided exactly or it won't work correctly.
Hopefully once that finishes running you should get a BUILD SUCCESSFUL message. At this point, there is just one last command to enter, as follows:
This should also yield a BUILD SUCCESSFUL message,a t which point you're totally finished! You can move on to adding the Forge workspace to Eclipse.
I hope this helps! If the problem persists, make sure to let us know and we'll keep wroking through it.