If repulsive gravity exists, then instead of time dilation, as you would have for gravity which attracts, there would need to be time interval COMPRESSION associated with that, wouldn't there? By means of analogy, obviously. Once things get moving, the kinematic term dominates time dilation, but according to Guth's analysis, this effect is not even a consideration. Whatever else Guth had in mind, a second observer at rest with respect to inflation is not in there. It can't be, since inflation proceeds at superluminal speed.
Worse, inflation requires no second observer at rest in order to make its predictions about the way inflation proceeded with respect to time. You can't even do a relativistic analysis unless there is a second observer, that is unless you subscribe to the flawed ideas of autodynamics, a physics cult started by an obscure WWII era Italian physicist who immigrated to Argentina, and whose ideas about single observer relativity were carried forward for the next 40 years by lunatics and fellow Einstein haters like David de Hilster.
Yet Guth speaks of inflation as an expansion that is GEOMETRIC in LINEAR time (as in time which proceeds as if there were no time dilation at all). At least he realizes, it would need to start out at low density. So much for any semblance of energy conservation.
I see no justification for the idea of repulsive gravity in anything Guth says. We already know from experiments that antimatter exhibits the same normal attractive gravity which matter does. We already know that black holes with masses in the range of 100 trillion galaxies the size of our own seem to have had attractive gravity and have been sucking in surrounding mass for in excess of 10 billion years, in which case, the repulsive gravity of inflation would have prohibited its formation in the first place. Or does not attractive gravity interact with repulsive gravity at all? By means of this test, inflation theory already does not stand up to experimental and observational muster.
It doesn't work. No surprise, quantum gravity doesn't work either, or at least not as well as GR. Any theory which ignores time dilation or just time in general is not a likely candidate for a theory that explains much of anything. If you don't have a clue what time is, what really are the chances of understanding gravity?
Try again.