1988 Other GMC Models sudden pressure cold

i was checking my unit for coolant leak.it was cold
had not been started . i pump up pressure with
cooling system pressure tester to 16 psi.
looked over cooling system for leaks .
started unit observed pressure guage climbing to
25 psi. shut off unit released pressure to zero.
started unit again pressure did not go out of range
16 psi. possible head gaskets ?