
I've added some more points and an extra circle.
If you draw the two tangents from a point outside to any circle, the tangent lengths will be equal. So if G is the midpoint of AB, then GA = GP = GB and a circle centred on G will go through A, B and P.
So APB = 90 and so cos(ABP) = cos(x) = sin(PAB)
Also ABP = BPD by the parallels.
PB = ABcos(x) and PBcos(x) = 0.5PD => PD = 2AB cos^2(x)
Similarly CP = 2AB sin^2(x) => CD = 2AB(cos^(x) + sin^2(x) ) = 2AB
When a line is tangent to a circle (at P, say) and PQ is a chord of the circle, this chord makes an angle across the other side of the circle that is equal to the angle between the tangent and the chord. I have called this the 5th angle property of a circle (because there are 4 others)
So using the property ABP = BDP and also BAP = ACP. I know that APB = 90 (this is the 3rd angle property of a circle, same thread) so BAP + ABP = 90 => ACP + BDP = 90 as well.
So mark H = midpoint CD => AB = HD = CH => ABHC and ABDH are parallelograms.
By the 5th angle property of a circle ABP = BDP and BAP = ACP
So ACP + BDP = 90
triangles ACH, BHD and HAB are congruent (SAA) so AHB = 90 => H lies on the circle through A, B and G => GH = 0.5AB