Well, then you can’t simulate accessing a GPIO controller of a very different MCU this way. At least you can’t use/access physical addresses not existing on your Ubuntu box.
In addition Linux itself doesn’t allow direct access to physical addresses because it uses virtual memory/MMU and kernel/userspace address space split.
I don’t know what you’re trying to do, but you could use e.g.
pGPIO_REGS = (GPIO_regs *)malloc( sizeof(GPIO_regs) );
to get some accessible memory and initialize the structure accordingly to test/simulate your code up to a certain degree.