Gap fiber Bragg grating (g-FBG) exhibits both Fizeau interference and phase-shifted fiber Bragg grating (PSFBG) spectrum. Their different sensitivities to micro-gap and temperature are demonstrated respectively, based on which a micro-gap and temperature simultaneous measurement method is proposed. The reflective spectra with different micro-gaps are tested by g-FBG experiments, which fits the simulation results. Data analysis shows that the micro-gap measurement error is less than ±5 nm. A g-FBG based sensor is made and simultaneous micro-gap and temperature measurement is obtained. Temperature measurement is achieved with a sensitivity of 8.3 pm/℃ and a low error of 0.1 ℃. This proposed g-FBG based simultaneous micro-gap and temperature measurement exhibits the advantages of high-accuracy, compact size, and flexible designing. With the related equation between temperature and micro-gap, temperature-induced gap change can be compensated and the temperature-independent micro-gap measurement is able to implement.