I’m not sure if GDevelop has added in features for mobile resolutions that I know of, however, you can change the resolution of the window. What I would do is probably make GDevelop detect if the device is a mobile device and then make the resolution difference. Then add the option to change the resolution to a range of different resolutions.
The problem is not realy the resolution, more the format 3:2 / 4:3 / 5:4 /16:9 / 16:10 and so on. At this moment, you can’t make a game, that fits in all display formats :-/ Or is there a trick ?