Configure GPIO pin for infrared interface (IRTIM) of STM32F030x4/x6/x8
The actual register isn't named GPIOx_AFRx, the "x" is a placeholder for other designators.

In this case, the first "x" is a placeholder for a numbered general purpose I/O, and the second "x" is a placeholder to indicate the alternate function low or high register.

If you look at sections 8.4.9 and 8.4.10 (PDF page 128) of the document you linked, you'll find they deal with the GPIOx_AFRL and GPIOx_AFRH for the low and high registers, respectively.