Inside Google docs, what is the same thing as Sans Serif font style inside Gmail?
I am not quite sure if I understand the question.
When you write a mail, you have sans seriff chosen as default.
Since sans seriff is just a family of fonts, it simply means “whatever software the recipient of this e-mail uses to open his/her mail, choose the default sans seriff font you have to show this text”.
Most of the time, this default font is Arial which is one of the few fonts that are available in any operative system and I would say that 99,9% of the time, if not 100%, it is the main sans seriff font for this reason.
But if Arial wasn’t the default sans seriff for that device, then it would choose the first sans seriff font available.