Google for how to send email through gmail, this is very well documented and every problem you will face (and there will be many) has already been answered. One issue I see is that you set UseDefaultCredentials to false after you've supplied the credentials. I know it sounds weird but SmtpClient class is not well written :) you need to set UseDefaultCredentials to false before setting the credentials themselves (you're also setting it after calling .Send).
SMTP.UseDefaultCredentials = false;
SMTP.Credentials = new NetworkCredential()
{
UserName = "<removed>",
Password = "p@ssw0rd-r3m0v3d"
};
SMTP.EnableSsl = true;
SMTP.Send(PassRecMail);
There may well be other issues, eg you have to configure your gmail account to allow people to send programmatically, you might be using a different "from" address to the credentials you supply. Whatever it is, sending through gmail is an incredibly frequently asked question so just google for the code and config required.
My advice, though, is to simply not send through gmail at all, send through your webhost's SMTP server instead, and for local testing use your network's SMTP server or use something like smtp4dev.