Keep getting “imap.gmail.com” pop up can’t find server.

This means that your gmail mail account cannot contact the server to check for mail. You need to check your account to see what the issue is. It could be a username or password issue. If you have added gmail to your native Mail app, then go to Settings>Passwords & Accounts, click on gmail and check settings. If you are using the gmail mail app, then check those settings. The easiest way to correct it if it is in the native Mail app is to delete the account, power cycle the iPhone, then add it back.