If you have trouble logging in gmail account with Apple mail after Mojave 10.14.4, try to remove the following from Keychain access.
“https://t.co/aSukyA2zEf.oauth-expiry-date”
“https://t.co/AGKstpPOsh.oath-refresh-token”
“https://t.co/AGKstpPOsh.oauth-token”