Cannot decrypt GPG file after installing GPG Suite from GPG Tools
Assigning Files

Your Mac should have an association with a *.gpg file. 
The appropiate service is named GPGServices.service and can be found in /Library/Services/. To manually assign the file extension to a application or service click on the myfile.gpg and then CMD+I.

Now you can chose a application with which the file should be open with.

If you click and hold the drop down menu, chose "Other ..." and a dialog will open. Click CMD+Shift+G and type /Library/Services.
Chose GPGServices.service, change for all and close the Info window.
If you can't find GPGServices you may have a corrupted or old installation. In this case install the GPG Suite again.

Decrypting with a terminal

To decrypt the file you can also use a Terminal. 

Open Spotlight and type "Terminal.app"(or open a Finder > Applications > Utilities > Terminal.app) and click on it. You will see the command line.
Type gpg -d(mind the space after -d) and drag the file into the terminal. Hit enter and you will be asked for you password.

Example

gpg -d myfile.gpg