Add the gin, gentian liqueur, and Lillet Blanc to a mixing glass.
Add ice and stir until well chilled.
Strain into an old-fashioned glass and garnish with a twist of lemon.
Notes
Stir, never shake. Stir for at least 30 seconds for a well-chilled drink. Since this has no "mixer", it will only taste as good as the ingredients you use, so make sure to use quality ingredients.