It says it is downloading and then pretends to make changes on my computer but just goes back to the download process again after that.
Is GarageBAnd stuck downloading the additional content or the application itself?
if it is the additional content, try the solutions in this thread -
A manual download using the Terminal as mteep suggests:
Generally, with problems when istalling, make sure you are running the installer logged in from an account with administrator privileges, and also disable the energy saver in the system preferences, so your mac will not fall asleep while installing and downloading.