I have a site that is not doing well on search and I don't know why. My site doesn't have the GDPR popup, will that be an issue with Google rankings? If so, how does Google know?
To my knowledge, Google has never announced that GDPR compliance is a factor in their ranking algorithm, so it's reasonably safe to assume that your ranking issues stem from a different cause.
However, GDPR compliance can come into play if you integrate Google products into your website, for example AdSense.
For a general overview of ways to increase ranking, see:
What are the best ways to improve a site's position in Google?