Using the GPIOs of an STM32 Discovery board
Is it possible to use the GPIOs without interfering with the internal goodies? I mean, if I use a GPIO pin which is also used by the LCD, can it be a problem?


Yes. Unplug the LCD glass from the board -- it's mounted in a DIP-style socket. Once you've done this, the pins are available for use.

There are a few other peripherals on this board which cannot easily be disconnected from the target MCU. (In particular, the user LEDs LD3 and LD4 cannot be disconnected without some desoldering.) You should have plenty of GPIOs to work with already, though.

The pin planner tool in STM32CubeMX is incredibly helpful in selecting pins for your application. I would highly recommend that you use it.