Pumping your creatures by 3 toughness to survive a Toxic Deluge is also pretty epic
This is the main reason Veil of Summer is less relevant in Legacy as well its just not as good when counterspells are more mana efficient, as they can just cast another one to stop the Veil itself
There are tons of other fun cards in this set and some interesting Commanders to build around, but this list should make up some of the must haves for Commander players
Abaddon can give spells cascade, as does the unpleasantly-named Sloppity Bilepiper
All of this is to say that, for this precon, Gandalf may not do it justice as a Commander
Once in play, a land can be tapped -- used up for that turn -- to generate a mana of that color, and mana is used to play the rest of the cards in your deck