Gruber's painting is based on color. His canvases have different color tonalities, where the inclination to one or two colors that are the bearers of the atmosphere prevails. The atmosphere of specific places is central to Petr's paintings and unique on the contemporary art scene. Petr paints pictures from memory without a template and hardly ever uses photography. The motifs refer to the places she visited and to which she repeatedly returns in her paintings. Peter's paintings often capture specific moments, very fleeting because they often capture a moment of light in a scene that disappears in a moment in a real place. In Gruber's paintings, light is also an element that abstracts the painting, and his painting is precisely on this boundary, where everything is not described. Gruber works with a mystery that is often anchored in an event that takes place on location. Often this event - an action, a group of people - is indicated only by a hint. However, this hint can be very clear to the audience. In the distance, the lights of houses shine in the darkness of the evening, the orange sky shines through the dark forest, or people have just gathered in tents, and the scene of the painting is suddenly swallowed by the melancholy of the coming evening.

The author's scenes are often deformed in shape, and the compositional game works with the corner of the picture or the side of the canvas, where the motif often just peeks into the scene. It thus enhances the drawing into the image. The canvases are often vertical, which supports the shape of slopes, high hills, etc., which will therefore be even longer. Working with the subconscious is important to Peter, and the places he paints are not a direct transcription of real reality, but are rather the very idea that stems from the subconscious and from his own imagination. They are connected with nature, which is close to the author and has a spiritual dimension for him. Here we can observe a mountain hotel on the shore of the Black Lake, which is not really there, or the lights of a mountain road, which are not really there. The point is that the author feels them in the memory and thus creates personal reflections of experiences that in complexity create the author's registration map of feelings and pictorial visions, often on the verge of a dream, melancholy and contemplative relaxation.