Find the angle between the plane of a triangle and the longest diagonal in a cube

Here is the question of the video: EFGH-ABCD is a cube. HA is a diagonal in the plane HDAE. AC is a diagonal in the plane ABCD. CH is a diagonal in the plane CGHD. GD is the longest
diagonal in the cube. What is the angle between the plane of the triangle HAC and the line segment GD?