Add all ingredients into a cocktail shaker without ice and shake it for 20 seconds. That will help the egg white build the beautiful foam. (If you don't like egg white, you can start with the next step)
Put ice in the shaker and shake again. To properly cool the drink and keep the foam, shake it for around 10 seconds.
Strain into a chilled sour or coupe glass.
Garnish your drink a few drops of Angostura bitters or with a lemon peel if you want. Voila, here's your perfect Gin Sour.