This log appears when logs were deleted on the device.
Here is why: the SDK queues logs locally on disk before sending them to the server, so that you can keep logging even if the mobile device is offline or disabled. However, there is a limit of 5MB in order to prevent filling up the disk.
Once the maximum disk size has been reached the oldest logs are deleted to keep the disk size within the configured limit. A bf_gap_log is written in order to signal this has happened, the message also includes time frame covered by the discarded logs.
This happens usually when the device logging is disabled in Bugfender or the mobile device was offline for an extended period of time.
If you see these logs happening very fast in the middle of your regular logs, it might mean your device is sending lots of logs in a short period of time, filling up the 5MB disk limit very quickly. You can increase the limit if you feel this is the problem (see setMaximumLocalStorageSize in your SDK reference).