GTK screen updating

General Usage Performance
1

My question

How does a person update the screen in gtk for a game or video updating?

notes

Glade was used to make the following code and I was wondering how I would update the screen (the ppm that is currently being used is just a placeholder)
this is the relevant image class that i wanted to update (about 60 times a second at 1920 by 1080 pixels)

<object class="GtkImage">
                    <property name="width-request">100</property>
                    <property name="height-request">80</property>
                    <property name="visible">True</property>
                    <property name="can-focus">False</property>
                    <property name="pixbuf">image.PPM</property>

how would I update the image?
This is the code I am using for the placeholder image

function GHW()
            nx = 1920 - 200
            ny = 1080 - 200
            open("image.PPM", "w") do image  
                        write(image,"P3\n" , string(nx) , " " , string(ny) , "\n" , string(255) , "\n")
                        for i in 1 : ny
                                    for j in 1 : nx
                                                r =  (nx - j) / nx
                                                g = i / ny 
                                                b = (i*j) / (nx*ny)
                                                ir = string(Integer(255 - trunc(255.99*r)))
                                                write(image, ir , " ")
                                                ig = string(Integer(255 - trunc(255.99*g)))
                                                write(image,ig , " ")
                                                ib = string(Integer(trunc(255.9*b)))
                                                write(image,ib , " ")
                                                write(image, "\n")
                                    
                                    end
                        end
            end

            nx = round(Int,nx / 3)
            ny = round(Int,ny / 3)
            open("mini_image.PPM", "w") do image  
                        write(image,"P3\n" , string(nx) , " " , string(ny) , "\n" , string(255) , "\n")
            
                        for i in 1 : ny
                                    for j in 1 : nx
                                                r =  (nx - j) / nx
                                                g = i / ny 
                                                b = (i*j) / (nx*ny)
                                                ir = string(Integer(255 - trunc(255.99*r)))
                                                write(image, ir , " ")
                                                ig = string(Integer(255 - trunc(255.99*g)))
                                                write(image,ig , " ")
                                                ib = string(Integer(trunc(255.9*b)))
                                                write(image,ib , " ")
                                                write(image, "\n")
                                    
                                    end
                        end
            end
end

If possible please help change the size of the image dependent on window size too that would be great