I had this issue with the iphone 6 plus - 128gb gold at&t. I imagine its an ios 8 issue itself and the transfering of an actively logged in app, which came from the backup. Anyways, Glad this fixed it because it was really annoying.
I followed the instructions but now am NOT able to login. Can you please help?
Sorry for the frustration! Based on the reports we're getting it seems there's something going on with the download process from the App Store.
Could you completely kill the app, uninstall GasTracker, hard reset your device (don't worry, no data loss by doing this) and then download GasTracker again.
To hard-reset, press and hold both the Sleep/Wake and Home buttons for at least 10 seconds, until you see the Apple logo.