Gmail ads are unique in the Google ecosystem because they require two clicks: one click to open the ad like an email, and a second click to the website. Your creative needs to be on-point in two places:

Failure to realize the two-step thought process for Gmail ads is the most common mistake I see. Think about the user. Email is a very personal space that you’re intruding on. Your ad is sitting there among chain emails from Grandma, updates from groups they’re a part of, and marketing emails from The Gap. If you’re having trouble succeeding with Gmail ads, your creative is busted.