Gatineau is the Quebec city facing Ottawa across the river, and it works best when you think of it as several former towns stitched together. Hull is the urban, museum-heavy sector closest to Parliament Hill. Aylmer is quieter and older, with a riverfront village feel. The Gatineau sector itself sprawls east, while the green hills of Gatineau Park rise just to the north.
For visitors, Gatineau's great advantage is contrast. You can spend the morning in the Canadian Museum of History, look back across the Ottawa River from Jacques-Cartier Park, and be walking around Pink Lake or the Mackenzie King Estate before lunch. It is less polished than Ottawa, more bilingual than much of Quebec, and often better value as a base.
Do not expect one neat downtown. Gatineau is a car-and-bus city with good cycling paths along the river, strong pockets of interest, and a lot of ordinary suburb in between. Use it for museums, park access, river views, and the easy cultural shift that comes from crossing a bridge from English-speaking official Ottawa into French-speaking Quebec.