Carlos,
Request you to take a look at TivaWare/driverlib/gpio.c - There is a function called GPIOPinWrite that does exactly what you want. The first parameter is the GPIO port base address, the second parameter is bit mask for the pins that you want to modify, the third parameter is the byte that you want to write to the masked bits.
In your case, GPIOPinWrite(GPIO_PORTA_BASE, 0xFF, 0xAB) will write the value 0xAB to the entire PORTA - in one go as you desire. You will have to configure the pins as outputs before this (using GPIOPinTypeGPIOOutputfunction).
Regards,
Shashank