I suspect that you will have to use Apple's Mail app as that is the only program that will allow you to "Open with" the attachments. I would doubt that a 3rd party gmail app can do that.
If you are getting a winmail.dat file, it sounds like you are probably using Outlook. I don't really know how to fix that, but you will have to make Outlook not encapsulate the file in that stupid winmail.dat format. You will have to Google on how to turn that off. You need to have an attachment in the email that ends in ".iif". That is the only way that you will be able to get it to open correctly in Time Master.