Here is a link you may be interested in.
http://blansett.tripod.com/patdata/g0000021.html#I479Like you, I am trying to figure out the Gilbreath line. I am a descendent of Michael M Gilbreath, son of David and grandson of Hugh and Elizabeth Johnston. It appears there is some confusion to Davids marriage so I am thinking he probably was married more than once. I look forward to any information on the Gilbreath line.