It's just a HAL and an IDE, with a truckload of user/third party supplied libraries for various modules, sensors, etc.
Plus, every sizable MCU/dev-board vendor supplies a Arduino HAL implementation (so called Core) for their board/mcu/module (or it's done by enthusiastic community).
The point of the GP was to refute the claim that they were started “before open source was cool”.
It's Atmel that derives massive benefit from GCC, or whoever implemented AVR backend for GCC.
Arduino doesn't - strictly speaking - depend on GCC, it could (and does) use any toolchain that is supplied by MCU vendor.
And it just happens so that many MCU vendors do often use GCC as part of their toolchain. Arduino just bundles that with vendor supplied tools for flashing, etc, like avrdude.
Which is to say - it's the MCU vendors that derive the main benefit from GCC.
Arduino will just happily use whatever toolchain MCU vendors provide.