Juniper is the only botanical which is in all gins. The cones of the juniper bush (often referred to as “juniper berries”) are required by legal statute, to be present and perceptible, in order for a spirit to be called gin. Juniper is in 100% of spirits that are designated as gins.
The juniper berry is known for imparting the traditional pine note of gin, although it can also come across as resiny, waxy, herbaceous, or even green and fresh.
Beyond the spirit and the juniper berries, you can feel free to experiment with whatever botanicals you like. Some of the most commonly used gin botanicals (after juniper) are coriander seed, angelica root, citrus peel and cassia bark or cinnamon – but feel free to raid the spice cabinet and herb garden. You could try bay leaves, rosemary, liquorice powder, lavender, cardamom, cumin, fennel seeds, lemongrass…pretty much anything you fancy!
Check out this link for some info on ratios and some recipe ideas. https://www.lovebrewing.co.uk/guides/gin-recipes-using-botanicals/