1. Download the Chrome extension CloudCodes Insertable
2. Reload your Gmail
3. And then after copying your table from Excel, click on the new icon on your gmail:
4. As soon as you click on this icon, your table will be pasted in the Gmail without losing its format
If you don't want to add an extension, you can do the following:
1. Copy Excel table into MS Word
2. Now copy this table from MS Word and paste into Gmail
The first option is required only if you want to do it straight away from Excel to Gmail by skipping a step.