I have found out how to use the fonts within the M5GFX library;
#include <Arduino.h>
#include <M5GFX.h>
M5GFX display;
void setup() {
display.begin();
display.setTextDatum(middle_center);
uint16_t x = display.width() / 2;
uint16_t y = display.height() / 2;
display.setFont(&fonts::FreeMonoBold24pt7b);
display.drawString("Test", x, y);
}
void loop() {
}