Tableau has been pretty open that version 1 doesn't have all the connectors Desktop has and that they will be adding more connectors in subsequent versions. I'm sure Google Sheets is on their radar, but the best way to bump it up in the priority list is to create an idea and rack-up the votes. Already the community has a bunch of ideas for numerous connectors that various members would find helpful:
Hadoop Hive: https://community.tableau.com/ideas/8698
Hadoop Impala: https://community.tableau.com/ideas/8678
Thanks for the info. I'm reading this and hearing that the plan is to slowly add connection methods over time. This makes this tool a shiny veneer but not terribly useful. This version seems to have been released out of beta prematurely.
Thanks for the question! As Joshua mentioned, we will be adding additional connectors with later versions of Tableau Prep. We want to know which connectors and features are important to you so please continue to ask your questions and submit your ideas.