The "Gray-box testing" methodology enforces what kind of restriction?

Only accessing the external operation of a system with no internal knowledge is the definition of black-box testing, not gray-box testing.

Gray-box testing grants the tester limited, partial access to or knowledge of a system's internal workings - such as some architectural diagrams or partial source code - without full visibility. This partial restriction defines the methodology and simulates realistic attack scenarios where a threat actor has some insider knowledge, making it more targeted than black-box testing while still retaining an element of uncertainty about the full internal design.

Having knowledge of only the internal operation while being unaware of external system behavior does not correspond to any recognized penetration testing methodology.

Complete and full knowledge of a system's internal operation describes white-box testing (also called crystal-box or clear-box testing), not gray-box testing.