Gazelle's App is designed to auto cache your login credentials so you can be auto-logged into the app. However, when some phones get low on memory they start auto deleting non-essential memory caches to free up memory necessary for critical system functions. This is especially an issue with all versions of the iPhone.
What do I need to do?
The solution is simple. Delete photos, videos, and old apps you are no longer using (in that order). Once you free up enough space your phone will once again remember your login credentials.