Once upon a time, when we were entertaining a few friends one weekend (pre-Covid), I was thrown for a loop, but only briefly. I was making a round of old-fashioneds for everyone when one of our guests politely informed me that they would pass because they didn't like bourbon. Okay. Well, I knew that this person liked cocktails and didn't want them to fill left out. I paused for a moment racking my brain for ideas. We always keep a full bar but I didn't want to have to play bartender all night mixing up different types of drinks. So I asked their thoughts on gin. Luckily they had a fondness for it and I immediately offered them two different options for a gin old-fashioned.
The first option contains Campari for a little bitterness while the second one is a straight substitution of gin for bourbon. Honestly, you can really get by using any standard liquor base in an old-fashioned. And you can play with the flavor profile by using different citrus peels (orange, lemon, grapefruit, lime), different sugar syrups (honey, brown sugar, molasses, demerara, maple syrup, agave) and lets not forget the numerous range of bitters currently on the market. As long as you know your basic, classic recipe, you are always able to make someone the perfect cocktail. 
His Ingredients - Gin-Campari Old-Fashioned
1 tsp simple syrup
1 tsp Campari
few dashes orange/grapefruit bitters
pinch sea salt
2 oz gin
In a rocks glass, muddle all the ingredients, except gin, together. Add the gin and some ice.
Her Ingredients - Gin Old-Fashioned
2 oz gin
1/4 oz simple syrup
2 dashes Angostura bitters
Citrus peel, optional
Stir all ingredients together in a rocks glass. Add ice, stir. Garnish with a citrus peel if desired.