It is the Alilaguna- very simple- walk to dock at airport, buy your ticket, board. I believe Gritti is right at the SM Giglio stop
Here is info w/maps
https://www.alilaguna.it/en
or take the ACTV bus to Piazzale Roma then Vaporetto #1 to SM Giglio
Bus info:
http://www.atvo.it/allegati/aeroporti/linea_35_dal_31.03.19_al_26.10.19.pdf
Vaporetto map:
http://actv.avmspa.it/sites/default/files/attachments/MAP%20OF%20THE%20SERVICES%20ACTV.pdf
Or
a private water taxi for $$$ (no taxis on wheeling venice !)
You might also check the Gritti website or email them they can tell you how to get there.
The Alilaguna would be easiest- no transfers. 15 E
Bus is 8 E plus vaporetto entails a transfer
Water taxi most expensive.
We bought our vaporetto pass at the airport and took the Alilaguna- we stayed very close to the Gritti- it was super easy- no steps or bridges to cross and then we had our vaporetto pass in hand on arrival.