Alas there's no progress bar for this or any way to see it directly.
However, there is a way you can work it out with a reasonable accuracy.
There's a program called NetCounter, which keeps track of your data usage. What you could do is make a note of your current data counts (or reset the counter to 0) and also find the size of your attachment. Then when you send your large attachment you know roughly when it will have finished sending because your data counter has increased by just over the size of your attachment.
Attachments are encoded into 7-bit for sending in emails, so they actually require more data to send the email than the attachment does in its original file format. I'm not sure if this is done by the GMail app, or by the Google Servers. So what you would need to do is send an attachment of a known size (say 1MB) and see how much data this requires to send by looking at NetCounter. You may find it's about 1MB still, or you may find it's up to 1.3MB.
