In a cocktail shaker, combine the gin, lime juice, simple syrup, and bitters. Add ice and shake well.
You want to shake until the cocktail shaker feels very cold, almost frozen, to your hand. This will ensure your cocktail is not only well combined, but it also chills and adds just a little water. I love it when a gimlet, martini, etc. almost has little icicles in the drink at first, and you get that by shaking well.
Strain into a coupe and garnish with celery if using. Enjoy right away.