The nice thing about using the gradlew script that comes with the project is that you won't need to have Gradle already installed. That script will take care of getting Gradle for itself. (The idea is that a user isn't forced to download/install Gradle to build a Gradle-built project.)
That said, there's still a little bit of setup you'll need to do to build the project. The build depends on some build code in a submodule that is shared among other Spring projects. Per the readme.txt file you are supposed to clone the Spring Social repository with a --recursive switch to get both Spring Social and the submodules. Of course, that information comes too late if you're reading the readme.txt file after cloning without the --recursive switch.
So, to get the submodule, do this from the cloned Spring Social project folder:
git submodule update --init
That will pull the submodule into a folder called buildSrc. At that point, ./gradlew should work fine.
I've updated the readme.txt file with these directions.
Spring Social Project Lead