I see a lot of "I want to learn more about ARM programming" threads, but the truth is 99% of programming on ARMs is nothing to do with the ARM CPU, it's all about peripherals or just plane C code.
gpio_set_pin_high(LED0_GPIO) is no closer to the hardware than digitalWrite() in terms of learning anything, it's just another name for a function to set a pin. As noted, even if you get down and dirty with the port control registers that's nothing to do with "ARM".
You could start writing code in assembler, that really is learning ARM. Apart from that offhand I think the only ARM things you can play with is the NVIC or the systick counter.
Rob