How to setup a chain Linux->UART->GPIO_0 on de10-nano?

You can connect the RX and TX to any of the 36 user pins GPIO header by referring to the user manual, make sure to assign the exact FPGA Pin Number according to the DE10 nano user manual on the page mentioned, in the Assignment Editor in Quartus.

GPIO is the General Purpose Input Output, the DE10 nano board has two 40-pin expansion headers. Each header has 36 user pins connected directly to the Cyclone V SoC FPGA. It also comes with DC +5V (VCC5), DC +3.3V (VCC3.3), and two GND pins.

You can refer to the DE10 nano user manual on page 27, 28, and 29 for the GPIO FPGA Pin Number;

https://www.intel.com/content/dam/altera-www/global/en_US/portal/dsn/42/doc-us-dsnbk-42-4302081511597-de10-nano-user-manual.pdf