After a reset, usually all port pins are configured as I/O input. Depending on the device, they are either high-impedance or have an internal pullup active. This is interpreted as high by most circuitry.
If you want the signal to stay low across a reset and only high if you program it through I/O, you'll need to attach a pulldown resistor to the pin that holds the signal low until the application software switches the I/O pin to low output. Also, first program the PxOUT register, then the PxDIR register, so you're not driving the pin actively high after setting it to output, until it is set low a few clock cycles later. (the register names are from the MSP series, but I guess the DM works similar in this aspect)
All of the PCI-muxed pins(GPIO[0-7]) do not have PULL UP capability, only PULL DOWN capability, but NEVER enabled. And it's missing the internal pull-up/downs shown in the datasheet. You will need to use external resistors where needed in your system.
This means that with no EXTERNAL pulls on these pins, they will be floating when configured as inputs. Depending on process variation, temperature, voltage, etc. these pins could potentially float LOW (logic 0) in some cases, or floating to a HIGH value.. Therefore, it is recommended to use external pull resistors on these pins if they are configured as inputs with no external source actively driving them.