Fennel, though often mistaken for anise due to it’s flavor, is not anise. It does contain anethole, which lends star anise its specific flavor. But it’s botanically different. The bulbs contain smaller amounts of the aromatic and are often eaten as a vegetable. In gin we often see the fennel seeds being used for their slight anise like flavor. It’s milder and slightly more vegetal, though often without being told, gin drinkers might think of it the flavor note as anise. In Indian culture, the seeds are often eaten after a meal as a breath freshener, or used as part of a blend of ingredients to create a licorice powder [perhaps leading to another common confusion point].
Common in absinthe, Fennel has become more common, especially in contemporary style gins that emphasize an herbal profile.
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/