Is this already a cocktail? 1.5 oz of gin, 0.25 oz sweet vermouth, 0.25 oz dry vermouth, 0.5 oz olive brine.

It sounds like you're describing a cocktail with a mix of gin, sweet vermouth, dry vermouth, and olive brine. This combination is quite similar to existing cocktails but with its own unique twist. Let's break it down a bit.

One of the cocktails that closely matches your recipe is the Dirty Martini, known for its vodka or gin base combined with vermouth and olive brine. However, your recipe includes both sweet and dry vermouth, which adds a layer of complexity not typically found in a standard Dirty Martini.

Your concoction bears some resemblance to a hybrid between a Martini Perfect and a Dirty Martini. The Martini Perfect, with its blend of dry and sweet vermouth, would impart a balanced herbal and slightly sweet flavor. Adding the olive brine introduces the savory, umami dimension akin to a Dirty Martini.

So, while your cocktail mix isn’t a classic by specific name, it combines well-known elements from established recipes to create a well-balanced, sophisticated drink. It's great for those who appreciate the nuanced flavors of both sweet and dry vermouth along with the briny kick from olive brine.