The best way I have found to save a Gmail message to you computer (in its’ original format) is to do this:
In Gmail, open the message you desire to save to your computer.
At the top of the message (right side) you will see a printer icon. Click on that icon.
When the “Print” window pops up, click on the drop-down box next to “Destination” and select “Save to PDF”, then click on “Save”, and you will be prompted to save the result to your computer. The saved file will be in the “PDF” file format.
Note: There is also another option on the drop-down menu where you can also “Save to Google Drive” that is handy when you do not want to save the file to the computer that you are working on.