To calculate the gravitational potential energy (GPE) of the 20kg mass 15m high, we can use the formula:

```

GPE = m * g * h

```

where:

- GPE is the gravitational potential energy in joules (J)

- m is the mass of the object in kilograms (kg)

- g is the acceleration due to gravity (approximately 9.8 m/s²)

- h is the height of the object above a reference point in meters (m)

Substituting the given values into the formula, we get:

```

GPE = (20kg) * (9.8 m/s²) * (15m)

GPE = 2940 J

```

Therefore, the GPE of the 20kg mass 15m high is 2940 joules.