Inside the app, you work with vectors which never lose quality but when exporting as an image it can be pixelated, depending on the quality of the export.
With Gravit Designer, you can do basic photo manipulation, but Photoshop or Gimp are definitely better suited for handling raster images. Gravit Designer is a vector editor, meaning that everything you create in the application can be scaled and manipulated without losing quality. So, it’s best to import SVG or PDF files, as they are vector-based.
Here’s a comparison between raster and vector: https://graphicmama.com/blog/vector-and-raster-whats-the-difference/.