The HTC One M8 is receiving a small update sized 25 MB, promising improvements in — what else! — performance and stability, and some bug fixes. The One M8 GPe users should start receiving notification starting today, as the rollout has already begun, and even if you converted you non-GPe M8 to GPe one by flashing GPe’s firmware, you would get the notification to update. The One M8 GPe is already on Android 5.1, and given the size of the update, it’s definitely not Android version upgrade by any means.
Of course, you need to be stock system, completely unmodified ever, and stock recovery for this. Otherwise OTA will fail. We don’t have the OTA available on download, but it shall arrive soon. Thanks Izick for bringing this up!