From Toronto's perspective, getting Gibson is not necessarily about what you give up to get him -- it's about what you have to give up to make space for him.
If Anaheim wants to retain $2.4m and take Mrazek back, then absolutely, you probably go and part with Liljegren, 1st rounder, Robertson, etc... but asking a team to retain on a player for 5 years, when they can likely move him elsewhere with no retention, is a fairly monumental task. It's also double what the Leafs retained on Kessel when they moved him.
If you're acquiring Gibson at $6.4m, even if you assume Mrazek going the other way, then practically speaking, Nylander has to go... probably not in the same deal as I don't think Anaheim is in a position to really benefit from Nylander, but he's the cap space required to do it.
edit: that being said... even if Anaheim is cool with the concept of Gibson at say, $4.5m for Mrazek at $3.8m + a haul... I think if you're the Leafs, you've gotta ask yourself. Would you rather have:
Gibson at $4.5m or Campbell at $5m plus the haul of futures you'd have to give up, minus the small asset you might have to attach to Mrazek to dump him.