efter enabling the mail scan i'm receiving error certificate in IOS for Gmail , can you please help

Hi,

try disabling smtps scanning, you should be able to trust your ios device for secure outgoing mail.

i'm sure it will be fix, but I need the benefit of the scanning, I need to scan all emails 

In that case you need to import you XG certificate into the ios devices.

How i can import the certificate for IOS ? 

Sorry, know the theory, but had to google to find the answer myself.

I search a lot and nothing, anyone can help here?

This one of many in google.

http://www.watchguard.com/help/docs/wsm/xtm_11/en-us/content/en-us/certificates/import_client_cert.html