I understand that this has been an issue for the desktop app. Unfortunately the work-arounds suggested by the more seasoned users doesn't seem to work for the iPad version.
Any brushes that are not Solid Pen will result in a visible gap despite the shape being closed/joined. The original gap will go away once you break the curve elsewhere (creating another gap). I'm under the impression that this is not actually a bug, but rather an aesthetic thing where textures appearing to be hand-drawn aren't supposed to look perfect. It'd be awesome to have the option to close the gap though.
Any feedback is welcome!