POWr Galleries currently are designed to display different sized posts, depending on what content you add to each entry. There is no automatic way to display a gallery as a grid.
However, you can create a grid yourself by uploading images that are the exact same size.
If you would like an option to make galleries display as a grid, vote here.
Feature Requests come from POWr users, and help us decide what Features to build next. Vote for this Feature, and we'll let you know as soon as it's ready!