How to fix this weird gray area on my object?
What I meant by weird gray area is the slightly darker gray triangle above the curved edges. This is a shading artifact caused by a non-planar N-Gon.
You will notice that when you right click and set Shade Flat, the shading issue goes away. Setting it back to Shade Auto Smooth, introduces fake roundness which makes your n-gon non-planar thus creating this shading issue. I think even adding more topology will not solve the issue because of few vertices used for the entire circular part of the cylindrical shape.
You need to start with more vertices for the cylindrical part from the get go to support this type of curved window shape. I would start with a circle with 192 vertices and extrude that into the cylindrical shape. Add
Loop Cuts for the top and bottom of window and then select its faces and use I to
Inset Faces to build a nice
Edge Loop around the window that will help for cleaner topology.
Add the supporting Edge Loops for the Subdivision Modifier. This can be tricky because we don't want the Edge Loop to go upwards because it will create a similar shading issue:
Instead you have to manually move around Edges and Vertices to make the Edge Loop go parallel with the other one. Don't forget to AM > Merge By Distance after you move vertices over each other.
Now you have very nice and clean quad topology: