Electronic – Using a GPIO pin as a power source

A complete answer to this question would be quite long. I'll give you some guidelines.

• Your serial flash may work under the SPI protocol. Most MCUs (including the one in your dev board) include hardware modules to communicate using SPI. If you use SPI to test your serial flash, you'll be able to benefit from the advantages that those modules provide: speed + robustness + less code to do the same + lower MCU utilization.
• The MCU in your dev board is an STM32F207IGH6, with UFBGA176 package. The datasheet for it, in pages 40 to 51, states correspondence between peripheral signals and pin numbers. The MCU has 3 SPI modules. Each SPI module involves 4 signals (NSS / SCK / MISO / MOSI). At least some subset of the signals of each one of them is already used in your dev board. This means that, to gain complete control of a full SPI bus, you need to remove some parts (like resistors), or open jumpers, in your dev board. Just because of this, and if the purpose is just to test serial flashes, I would recommend you to use another dev board. There are plenty that give you access to complete SPI buses.
• For instance, SPI1 is mapped to pins PA4, PA5, PA6 and PA7. Those are available at connector CN2 of your dev board. However, those signals are already used by some functionalities, in your dev board. If you want that SPI1 bus to connect your serial flash to the MCU, and nothing else, you need to remove R115, the camera module, R69, RS2 and open JP8.
• In the serial flash datasheet, read section "Serial Peripheral Interface Modes" and look at Figures 6 to 8, to see how you need to make the SPI connections. In fact, if you don't know much about SPI, best thing you could do first is to learn about it. It can't be explained in one paragraph.
• Yes, you can use GPIO pins, instead of an SPI port, but you will need to big-bang the protocol.

• GPIO_PuPd (Pull-up / Pull-down)

In digital circuits, is is important that signal lines are never allowed to "float". That is, they need to always be in a high state or a low state. When floating, the state is undetermined, and causes a few different types of problems.

The way to correct this is to add a resistor from the signal line either to Vcc or Gnd. That way, if the line is not being actively driven high or low, the resistor will cause the potential to drift to a known level.

The STM32 (and other microcontrollers) have built-in circuitry to do this. That way, you don't need to add another part to your circuit. If you choose "GPIO_PuPd_UP", for example, it is equivelent to adding a resistor between the signal line and Vcc.

• GPIO_OType (Output Type):

Push-Pull: This is the output type that most people think of as "standard". When the output goes low, it is actively "pulled" to ground. Conversely, when the output is set to high, it is actively "pushed" toward Vcc. Simplified, it looks like this:

An Open-Drain output, on the other hand, is only active in one direction. It can pull the pin towards ground, but it cannot drive it high. Imagine the previous image, but without the upper MOSFET. When it is not pulling to ground, the MOSFET is simply non-conductive, which causes the output to float:

For this type of output, there needs to be a pull-up resistor added to the circuit, which will cause the line to go high when not driven low. You can do this with an external part, or by setting the GPIO_PuPd value to GPIO_PuPd_UP.

The name comes from the fact that the MOSFET's drain isn't internally connected to anything. This type of output is also called "open-collector" when using a BJT instead of a MOSFET.

• GPIO_Speed

Basically, this controls the slew rate (the rise time and fall time) of the output signal. The faster the slew rate, the more noise is radiated from the circuit. It is good practice to keep the slew rate slow, and only increase it if you have a specific reason.