Here are the steps: Go through the attachment, includes the libraries built in for Google data, to send the Google Calendar invitation.
Step 1

public class eAppointmentMail
{
public string Name { set; get; }
public string Email { set; get; }
public string Location { set; get; }
public DateTime StartDate { set; get; }
public DateTime EndDate { set; get; }
public string Subject { set; get; }
public string Body { set; get; }
}

public class specified as input to my method "sendoutlookinvitation"

Step 2

include the following code in web.config under <configuration> section

<system.net>
<mailSettings>
<smtp deliveryMethod="Network" from="[email protected]">
 <network host="smtp.gmail.com" port="25" userName="[email protected]" password="myPwd" defaultCredentials="false"/>
</smtp>
</mailSettings>
</system.net>

Step 3

In your c# code try using the following snippet:

public static string sendOutlookInvitationViaICSFile(eAppointmentMail objApptEmail)
{
try
 {
Configuration config = WebConfigurationManager.OpenWebConfiguration(HttpContext.Current.Request.ApplicationPath);
MailSettingsSectionGroup settings = (MailSettingsSectionGroup)config.GetSectionGroup("system.net/mailSettings");

SmtpClient sc = new SmtpClient("smtp.gmail.com");

System.Net.Mail.MailMessage msg = new System.Net.Mail.MailMessage();

msg.From = new MailAddress(settings.Smtp.Network.UserName);
msg.To.Add(new MailAddress(objApptEmail.Email, objApptEmail.Name));
msg.Subject = objApptEmail.Subject;
msg.Body = objApptEmail.Body;

StringBuilder str = new StringBuilder();
str.AppendLine("BEGIN:VCALENDAR");
str.AppendLine("PRODID:-//" + objApptEmail.Email);
str.AppendLine("VERSION:2.0");
str.AppendLine("METHOD:REQUEST");
str.AppendLine("BEGIN:VEVENT");

str.AppendLine(string.Format("DTSTART:{0:yyyyMMddTHHmmssZ}", objApptEmail.StartDate.ToUniversalTime().ToString("yyyyMMdd\\THHmmss\\Z")));
str.AppendLine(string.Format("DTSTAMP:{0:yyyyMMddTHHmmssZ}", (objApptEmail.EndDate - objApptEmail.StartDate).Minutes.ToString()));
str.AppendLine(string.Format("DTEND:{0:yyyyMMddTHHmmssZ}", objApptEmail.EndDate.ToUniversalTime().ToString("yyyyMMdd\\THHmmss\\Z")));
//str.AppendLine(string.Format("DTSTART:{0:yyyyMMddTHHmmssZ}", objApptEmail.StartDate.ToString()));
 //str.AppendLine(string.Format("DTSTAMP:{0:yyyyMMddTHHmmssZ}", DateTime.UtcNow));
 //str.AppendLine(string.Format("DTEND:{0:yyyyMMddTHHmmssZ}", objApptEmail.EndDate.ToString()));
 str.AppendLine("LOCATION:" + objApptEmail.Location);
str.AppendLine(string.Format("DESCRIPTION:{0}", objApptEmail.Body));
str.AppendLine(string.Format("X-ALT-DESC;FMTTYPE=text/html:{0}", objApptEmail.Body));
str.AppendLine(string.Format("SUMMARY:{0}", objApptEmail.Subject));
str.AppendLine(string.Format("ORGANIZER:MAILTO:{0}", objApptEmail.Email));

str.AppendLine(string.Format("ATTENDEE;CN=\"{0}\";RSVP=TRUE:mailto:{1}", msg.To[0].DisplayName, msg.To[0].Address));
str.AppendLine("BEGIN:VALARM");
str.AppendLine("TRIGGER:-PT15M");
str.AppendLine("ACTION:DISPLAY");
str.AppendLine("DESCRIPTION:Reminder");
str.AppendLine("END:VALARM");
str.AppendLine("END:VEVENT");
str.AppendLine("END:VCALENDAR");
System.Net.Mime.ContentType ct = new System.Net.Mime.ContentType("text/calendar");
ct.Parameters.Add("method", "REQUEST");
AlternateView avCal = AlternateView.CreateAlternateViewFromString(str.ToString(), ct);
msg.AlternateViews.Add(avCal);
NetworkCredential nc = new NetworkCredential(settings.Smtp.Network.UserName, settings.Smtp.Network.Password);
sc.Port = settings.Smtp.Network.Port;
sc.EnableSsl = true;
sc.Credentials = nc;
try
 {
sc.Send(msg);
return "Success";
}
catch
 {
return "Fail";
}
}
catch { }
return string.Empty;
}

It will simply download an ICS file, once you click on that it will open up with outlook calendar to show your appointment created.