Hi there WendySob,
Thanks for joining us in Apple Support Communities! We understand you're seeing an alert on your iPhone indicating the server can't be verified for 'imap.gmail.com', and this only appears when your iPhone isn't connected to Wi-Fi. We're glad you're here, and we can help.
To start, you'll want to be sure cellular data is turned on for Mail in Settings > Cellular. The 'Set whether cellular data is used for apps and services' section of this link provides more details:
View or change cellular settings on iPhone
Next, this appears to be an alert related to a Gmail email account in the Mail app. Let's start by removing the email account, checking to see what your server settings should be, then add the account again manually. To find out what your email account settings should be, enter your email address in the search field of this link:
Mail Settings Lookup
Afterwards, try adding the account manually. Have a look at the support article below to learn how. You'll want to specifically take a closer look at the section entitled, 'Enter account settings manually':
Add an email account to your iPhone, iPad, or iPod touch
Let us know if that helps.
Cheers.