There are various ways to get that result. Another way, still using a layer mask, but first you need to have a look at image sizes.
The map is quite small 490 x 512 pixels, You probably need to scale that up to suit the background image or maybe the other way, scale the background down to suit. Up to you. Assuming the map has to be scaled up
Open the background image
File -> Open as Layers the map, so it is on top of the background.
Use the scale tool to size the map, you can temporarily reduce opacity to see where you are.
Once scaled, tidy up with a Layer -> Layer to image size
The next bit works because the map is black and white.
With Map layer active Layer -> Mask -> Add layer mask and select Greyscale copy of layer.
and that is about it: Just get all that in a one minute example : https://i.imgur.com/960fElt.mp4
The map is quite small 490 x 512 pixels, You probably need to scale that up to suit the background image or maybe the other way, scale the background down to suit. Up to you. Assuming the map has to be scaled up
Open the background image
File -> Open as Layers the map, so it is on top of the background.
Use the scale tool to size the map, you can temporarily reduce opacity to see where you are.
Once scaled, tidy up with a Layer -> Layer to image size
The next bit works because the map is black and white.
With Map layer active Layer -> Mask -> Add layer mask and select Greyscale copy of layer.
and that is about it: Just get all that in a one minute example : https://i.imgur.com/960fElt.mp4