Wiring garage ceiling light receptacle to gfci outlet
Although Harper is partly correct a GFCI receptacle must be readily accessible and code defines this as 6’7” max exhibit 100.1 in the 2020 code this is also the max height for a disconnecting /circuit breaker switch handle as specified in 404.8.A.
The receptacle will need to be moved down or a GFCI circuit breaker installed in the panel to be code compliant.
The problem you currently have is your tester (many on this site refer to them as magic 8 ball testers because you are only sure they are correct when the circuit works other than that it’s a guess).
Have you tried pressing the test and reset buttons on the GFCI many models require this to turn on the first time and some models require it after every power cycle.