SMTP protocol doesn't return such data data in any way.
What you can do, is to use IMAP to select "Sent Itmes" folder and find the sent message.
I think the best way to do it, is to remember "Message-ID" header of the message before sending (IMail.MessageID) and using that in a search query.
using(Imap imap = new Imap())
{
imap.ConnectSSL ("imap.gmail.com");
imap.UseBestLogin("user", "password");
var common = new CommonFolders(imap.GetFolders());
imap.Select(common.Sent);
List<long> uids = imap.Search(
Expression.MessageID(email.MessageID));
Then use GetMessageInfoByUID or GetEnvelopeByUID to retreieve Gmail MessageId (Envelope.GmailMessageId)