Is it possible to launch this app using a custom URL?

Yes, you can launch the app using the following URL's:

• Launch the app normally: ContactsSyncGG://
• Launch the app and initiate a sync: ContactsSyncGG://sync
• Launch the app and initiate an auto sync: ContactsSyncGG://autosync

If using Synctastic rather than Contacts Sync, replace "ContactsSyncGG://" with "Synctastic://"