You can't just pick ports at random. A server process listens on a specific port for a connection to be made. In the case of mail, port 25 is the common default for connecting. For web traffic it is port 80, etc. To reduce spam proliferation, ISPs block the common ports for mail forcing the subscribers to send mail through the ISPs mail server via an authenticated account. If the ports to connect to gmail are blocked, then there is essentially nothing you can do other than use the ISPs mail server information.