Welcome to Android Central! What format does Tubemate download in, and where does it download to? Are you sure the format is one that the Gallery can recognize? If the videos download to a specific Tubemate folder (and not the Downloads folder), there might be a ".nomedia" file in that folder, which instructs the system's Media Scanner to ignore that folder when looking for photos and videos to put in the Gallery.
What do you mean it can play through tube videos download list?