There is gravity at the Hoover Dam, just like everywhere else on Earth. Gravity is a fundamental force of nature that attracts objects with mass towards each other. It is responsible for holding everything on the Earth's surface, including the water in the Hoover Dam reservoir.
If there were no gravity at the Hoover Dam, the water in the dam would float off into space instead of being held in place. The dam itself would also be subject to different structural forces, and the surrounding area would experience significant changes in atmospheric pressure, making it inhospitable for life as we know it.
The misconception that there is no gravity at the Hoover Dam might arise from the fact that the dam's walls are curved. This curvature is intentional and helps distribute the immense weight and pressure of the water to the surrounding rock and foundation. However, gravity is still present and necessary for the dam to function properly.