The bearing G is the angle between the north of the grid (Lambert Y) and the direction you want to follow. It’s an angle you can easily measure on a map - that is if you traced the grid - indeed there’s always a grid line not far from the route you’re following. It is used when one does precise orientation.

The geograpical direction AG is the angle between the geographical north NG (the vertical on a map) and the direction you want to take. It is the angle that’s used for basic orientation.

The magnetic direction AM is the angle between the magnetic north NM ( shown on a compass) and the direction you want to follow. That’s the angle you use when walking with a compass.

BE careful ! Orientation angles are always measured clockwise.

To switch from one angle to another without getting mixed up see this article.