Is it possible to upload a file with GPS values onto my arduino?
The Arduino does not have a file system per-se, to which you could upload files, unless you create the code that does that. There is "EEPROM" space on the Arduino processor that can hold 1024 bytes of data and survives power-cycles. That should be sufficient to hold 20 lon/lat pairs of sufficient precision.

As for how to get the waypoint data onto the Arduino, you could transfer them over the serial port from your computer, either by manual entry from your computer over the serial monitor of the Arduino IDE or by writing a script in Python or some other language to read the waypoint file and transmit the data according to the protocol you invented.

There are are also SD Card peripherals you can add to an Arduino, so you could potentially just load the waypoint file onto the SD Card from your computer, and then insert the card into the SD Card reader on the Arduino and have it read the data from there.