Malgus won because of his stealthed forces. Revan doesn't have that.
Malgus also wasn't quite as good of a tactician that Revan was...
Really though G0-T0 has this in the bag, the fact he has no real military forces aside from the HK-50s means that he quite simply won't show up for the major battles, while Grievous and Revan are slugging it out, G0-T0 will quietly be setting up the deaths of his opponents.
It's tough to take on an opponent that is operating by totally different set of rules than you are.