STM32 dev kit: Which power pin, APP or EMU

Note:
This answer was originally written before we knew anything about the receiver, like its 1.8V power supply. Telaclavo's answer is good. As a more general answer the PNP/MOSFET solution remains; you don't want to power parts of your circuit from a microcontroller's I/O pin.

You don't want to do that! You'd configure the pin as output and make it high to provide power to the GPS receiver, but microcontrollers I/Os can only supply limited current, 25mA for the STM32F205xx (Section 6.2 page 72 of the datasheet), which will be too low for powering your (and any other) GPS receiver (34 to 38 mA, as stated in the datasheet)

Use the I/O pin to drive a PNP transistor which will supply the required current.

Note that using a PNP inverses your logic: a logic low will turn the receiver on.

I would not use an NPN for this. In common emitter it would mean that the receiver's ground is a few hundred mV above ground, and a circuit should have one single ground which is the same for every component. In common collector you would lose too much of your 3.3V power supply.

edit
Wouter would use a MOSFET instead of a BJT, and that's a good alternative. Just make sure you choose a logic-level FET, which will give you enough current at a \$V_{GS}\$ of -3.3V. The Rohm RZE002P02 is a suitable type. It will also have a lower voltage drop if your receiver needs less than about 200mA.

edit 2 (re clabacchio's addition of a datasheet)
This device operates at 1.8V, the STM32 at 3.3V. You can use an LDO with an enable input and control that from your microcontroller. No transistor needed. (Thanks for the suggestion, markrages.) You'll also need level shifters for the data.

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.