In this paper, losses in a 600V Gallium Nitride (GaN) Gate Injection Transistor (GIT) were evaluated in different operation modes of a boost converter. Analytical loss model of GaN GIT, in which circuit and package parasitics are accounted for, was developed to assist the evaluation. Losses in GIT were assessed in a boost converter using the model and the results showed that: in Continuous Conduction Mode (CCM) and Boundary Conduction Mode (BCM), turn-on loss, mainly originated from discharging of transistor output capacitance, dominates in GIT; in Boundary Conduction Mode with Valley Switching (BCM-VS), where transistor is switched on with reduced voltage and zero current, turn-on loss can be greatly reduced. In BCM-VS, where turn-off current is higher than CCM and BCM, turn-off loss dominates as C,gd is large and the ratio between Cds and Cgd is small in low voltage range. Experiments were performed to validate the loss model at both 100kHz and 1MHz as well as to prove and demonstrate the loss analysis.