I think it is bleed door for the air intake. It is strange that this aircraft has only one instead of two on the F-22 and KF-21. See the red circled vents on F-22 and KF-21.Way too small to be a gunport.
The red one is to bleed the air from the intake to the gap between the fuselage and the intake. The yellow circled one is to bleed the air from the top side of the intake. Both are necessary and can not be merged into one as far as it is caret inlet.
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