People can, through sign language, express anything that can be expressed through spoken language so far as I know. So gestures can be fully expressive (there's enough "gesture-space" to build a language). Sign language, though, is derived from that spoken language, because it was developed to allow the hearing-impaired to converse with speakers. Sign language uses gestures mainly at the "word" level and the occasional phrase, though finger-spelling allow allows a signer to convey letters.
What would a gesture-based language look like if it developed organically in a civilization of people who do not speak and hear? For the sake of this question, assume that -- as with speech -- the gesture language develops well in advance of a written form (if a written form even develops at all): we have a civilization of people with no written or spoken language but they use gestures to communicate with each other.
Would the building blocks of the language still be words and grammar, or should we expect to see something very different develop (a la the analogy-based language of Darmok, though that specific example isn't feasible so please don't focus on that)?