The Guardians would be out of their depth with these two cosmic villains, but then so are the Justice League.
I have no idea what this story will be about - Darkseid and The Anti-Monitor going to war with the other for... what exactly?
The motivations appear basic in the extreme, one detail of interest is that while Darkseid is well known to the League they have no idea who The Anti-Monitor is. What they will make of this 'new' threat will be interesting to see, we know he has the power to destroy universes, they though won't appreciate the scale of the threat he poses.
But apart from a basic villain Vs. Villain conflict I don't see what the point of this story will be...
Will they be forced to take a side, as they realise that one future is worse then the others of they win out?