Avoid saying "in," rather, ask, "what else in gin flavored with?" Gin is flavored with botanicals. Botanicals are parts of, extracts from, and essences of plants. Other than juniper berries, the Traveling Wilburys and Highwaymen* of gin are, coriander seeds, angelica root, citrus peel, licorice root, grains of paradise, orris, cardamom, cassia, and nutmeg. I use that pop/folk/country/western metaphor because everything in gin, other than juniper, is best known for something else. But in reality, hundreds of botanicals, of many different species are used to flavor gin with a wide variety of techniques.
Gin also likes to obscure your understanding further by using botanicals for different purposes. While juniper is the core flavor, other botanicals are used to accentuate aromas, complementing and or buttressing juniper — I assure you, much of what you attribute to juniper is actually coriander or angelica. Other botanicals add flavors, but also lengthen flavors and change the way your palate perceives taste. To tap the most abused cliche in current news: gin is playing three-dimensional chess with your palate.
A favorite flavor, a complicated and tragic story, the Roy Orbison of gin for me, is nutmeg/ mace. The plant myristica fragrans bears fruit, the seed of which is nutmeg and the arils around the seed is mace. To remind you, plant: myristica fragrans and botanical: nutmeg and mace. It seems that few gins use the delicate, spicy flavors of mace, that is more commonly found in Christmas cookies of Chartreuse liqueur. Conversely, nutmeg is used less as a flavor but more of a way to extend flavors— you know, like how Roy Orbison ends a song.
The history of the second half of the last millennium is found in gin botanicals. I'm not here to write that book; I don't have the constitution for it. A historian may take a few thousand words or a whole book to explain it, but I won't: Dutch colonizers killed over 90% of the inhabitants of the small island from which nutmeg originates. I wasn't looking to just sprinkle a bit 'o death on this explainer; rather, I want you to take seriously, as I do. OK, bummer achieved, you earned your martini.
*sorry for the white dude metaphor, but I greatly look forward to the female foil(s) to these powerhouses, perhaps one headed by Beyoncé, another by Rihanna.