I recommend you the reading of Head & Meyer (2014) (attached). There is a companion website for the paper with stata code: https://sites.google.com/site/hiegravity/
There are different ways to construct the MRT in a gravity model. It depends on your data access and your decision as well. But I suggest you read Adam(2007) to find out some good tips about the MRT.
Thank you very much for replying to my question. However I am unable to download the attached paper. Can you please send me the paper or title of the paper so that I can find it ..