GSZ lets you put a toolbox of useful green creatures into your deck: you can put in Dryad Arbor to ramp for one mana
Then you sacrifice that copy at the end of the turn
The impending cost is pretty expensive if you never get to turn this into a creature, but thats kinda the point
The four drop 2/3 Azorius one will force your foes to cast more stuff for a mana each, and then your Azorius colored stuff one fewer
However, if you instead pay the one generic and one green, and also the kicker cost of two generic and one green, you will have kicked Gnarlid Colony, and its additional ability of entering with four +1/+1 counters comes into effect
The question that starts so many decks, both casual and competitive, is How do I maximize a specific card? Articles like the The Top 10 Modern Cards In Ikoria: Lair of Behemoths are designed to alert deckbuilders to the correct cards to focus on in the early days of a sets release