The gate drive voltage is supplied with the voltage on INTVcc. INTVcc is nominally 5V, but as this is supplied via an internal P-channel low dropout regulator from VIN, if VIN drops below 5V the voltage on INTVcc will be (VIN - dropout voltage).
For application circuits with VIN as low as 3V, it is important to choose MOSFETs that will fully turn on with a VGS voltage of about 2.6V as this is the UVLO threshold for INTVcc.