When I read this a question comes to mind:
Consider the following situation:
0: I have a Watchtower in hand.
1: I buy a Border Village (any on-buy effects are resolved, there are none).
2: I gain the bought Border Village (the way I understand it, bought cards are first bought, then gained. The gaining puts them in your discard, not the buying. On-buy effects are resolved BEFORE they are in your discard unless otherwise specified). The Border Village is placed on top of my discard pile (there are no on-gain effects that say 'instead').
3: Any on-gain effects are now triggered. There are two: the optional revealing the Watchtower, and the mandatory 'gain a cheaper card' on Border Village. Since they happen at the same time, I get to decide which one happens first.
World A:
4: I first do the Border Village effect. I gain a new card which covers up the Border Village in my discard, and since I only know the top card of my discard, I cannot top-deck Border Village with Watchtower. I can still reveal Watchtower to top-deck or trash the newly gained card, revealing the Border Village below it, but I don't know if this was the same Border Village as before. Furthermore, since I resolved the Border Village `gain' first, revealing the Watchtower can only refer to the most recent gain, and not one before it.
World B:
4: I first do the Watchtower effect. Since Border Village hasn't dissapeared to below the top card of my discard pile, or has been shuffled through my deck, I know with certainty that the top card on my deck is the gained Border Village. I reveal my Watchtower and top-deck it or trash it.
5: I now resolve the Border Village effect. Since Border Village was gained, the effect is triggered regardless of where it goes, so it being moved to the top of my deck or the trash doesn't remove its on-gain effect. So I now gain a new card and put it on my discard.
6: I can now use Watchtower to trash/top-deck the newly gained card as well.
So as far as I understand it, the problem with Summon is that it says "Gain. Set aside." Which is ordered, and not concurrent/parallel, so the actual gaining can mess up finding it when you need to set it aside, because other 'when you gain'-effects come before the 'Set aside'.