a few clarification questions to ask:
1) the goal of success? user aquisition? revenue (monetization)? market share growth?
2) the market of success? in US, or global?
3) customer group of success? Gmail to consumers or Gmail to enterprise custiomers?
4) in what time period of success? in a year? a month?
say if the goal is to measure if Gmail is success in US marketing on market share with consumers within a year. then the metrics:
# of new user registration to gmail
# of daily active users (users who registered also use gmail at least have 1 email per day)
# of gmail space have been used per user
# of users who use gmail over the past 12 month
# of users who stop use gmail after 1 months, 3 months, 6 months, 12 months
if this is for enterprise customers:
# of enterprise account registered
# of enterprise account last over 12 months
# of gmail space have been used in 12 months per account
# of enterprise accounts stop use gmail in 1 months, 3 months, 6 months, 12 months