what is “if it receives this mail” in this context ?
As already suggested with the IP cam binding you should be able to change an item without the need to go via mail.
In case you need to go via mail you need to poll the gmail server for new mails.
Question then is how often you would like to poll the server.
It should be possible / doable by using e.g. a python script to check the inbox for mails with a specific subject. The script could be executed e.g. via executeCommandLine or exec-binding.
Is email the only means of notification or do you have other options like sending http requests, ftp upload of alarm picture, SIP call, closing a contact, …?
Furthermore, does your camera provide a link where you can get alarm information from, like date of last alarm or latest alarm picture?