Add Pixel Watch 2 bought from Google Store to existing TMob line - Extra Charge?

Looks like it was a limited thing back in when they had their stores birthday.  Unless you bought the watch before Oct 3rd during that sale you won't get that data.

As part of the Google Store 25th birthday sale in the US, the LTE Pixel Watch is discounted and comes with “2 years of free data for your device.”

it's 250MB and you can't make phone calls and a slew of other things. If you want to use it as a STANDALONE you need a plan and that even include other carriers including Google Fi.