Gin,
a 12% glideslope means that for every 100 ft you move down the slope, you move 12 ft vertically downwards
To find the angle just remember that 12% = 0.12 an dthis is the sine of the glideslope angle.
This is because the hypoteneuse is the path you have taken and the opposite is the vertical distance you have moved downwards.
So the sine is 12/100 or 0.12.
So a 12% glideslope means going down a slope of 6.89 degrees below the horizontal.
Climb gradients are the same but (obviously) upwards!!
All of this assumes you are in still air.