PC does not recognize total RAM

My old (1965) Gateway P5-75 has 64mb RAM installed but on boot up and from every diagnostic tool and system info tool it shows 16mb. I do not know whether this requires a setting change to software or hardware. If software, what do I need to do?If hardware, how do I recognize and adjust the required setting? Gateway support was absolutley useless and actually blew me off with a “you touched the computer, we wont help” response.