​Does gin have juniper berries?

Yes. The flavour of juniper is one of the two key characteristics of gin (the other is a minimum level of alcohol). And the flavour of juniper comes from juniper berries. These fleshy modified female conifer cones are purple-back in colour when ripe; they come from the juniper tree or bush (most commonly, Juniperus communis).

The name 'gin' comes from either the French genièvre or the Dutch jenever (a precursor to modern-day gin, a spirit which is still made commercially and drunk) - which also mean juniper.

Apart from tasting of juniper, the other requirement is that gin must be at least 37.5% ABV (alcohol by volume).

So a juniper berry isn’t actually a berry at all - it just looks like one. Nevertheless it is the one absolutely vital botanical that most be used for a spirit to be described as gin.

And the predominant flavour of gin must be juniper. 

This is a very live issue as many gin liqueurs (and even some drinks claiming to be actual 'gin') cannot claim to have a predominant juniper flavour.

Added sugar and other dominant flavours like fruit and ginger can overpower the juniper.