In Sociocracy, when synthesizing proposal pieces into a proposal, how much leeway is there to fill gaps?

As much as the group allows. Generally, clarifications and filling gaps is a good idea to generate a good-quality proposal. It really depends on how the group perceives it. The chances of it being received with gratitude are higher if the person synthesizing the proposal is upfront about their additions and reasoning behind them.

For more information, check out our handbook Many Voices One Song section 3.3.2