Hi!
Well mayby not 40G ...that was a bit too much for a high winged trainer ...but in the eighties I heard some American Formula 1pylon airplanes that had been measured at 40 G in a tight turn.That was in the early eighties....with todays Dutch and Russian engines speed is much higher in international pylonracing....around 375-400km/h.
I would imagine that a trainer with a rear C of G traveling at around 100km/h , in a dive, pulling up sharply, could reach really high G forces.
Whether or not 20G or 25G it is vital that you reinforce the wing enough.
Having a wingspar is nice to have but 3-5 layers of 25g glasfiber cloth is also going to work.
I once flew off a wing during a sharp pylon turn at the nr 1 pylon with one of my P-51 F3D racers going around 300km/h ...the wing just broke in half ! That wing did not have a wingspar through the middle of the wing ...just 6 layers (different sizes) of 25g glass fiber cloth were used and some carbon fiber strings.

Regards !
Jan K