As Gravit is a vector tool, there isn’t some kind of eraser to remove some portions of an image, but that is still possible with other features, like Clip.
Create a shape with one of the drawing tools, outlining the portion of the image you wish to keep. In my example, I want to keep the leaf and remove the background.
After that, clip the image with the shape, creating a Mask shape. If you want, toggle the border’s visibility.
If you want the opposite effect, in this case removing the leaf and leaving the background, you will create a mask with a “hole”. Create a rectangle with the same size as the background and use the shape created with the drawing tool to create a Compound Shape, removing the shape from the rectangle.
Then you need to convert the resulting Compound Shape to a Path (or + + ) because Compound Shapes can’t be clipped. After that, clip the image to your just-created mask with the hole in the middle. Toggle the border’s visibility if necessary.