From K64P144M120SF5RM.pdf:
55.2.6, the definition for Port Data Direction Registers (GPIOx_PDDR) shows all reset to 'all zero', so that defines all GPIO-mode-pins as 'input' (high impedance).
The next question is 'which pins are GPIO at reset', and for that we jump over to table 10-2, and more particularly 10.3.1 'default' column, to see that 'most port pins' default to the analog-input modes, crystal, or disabled (ALT0). So, NOT GPIO, but high-impedance nonetheless.