It's certainly possible for excess wear to have occurred. This is because the transmission won't have been lubricated but will have been turning. That's why, if you have to tow a front wheel drive automatic, you lift the front wheels off the ground. If it's rear wheel drive you can disconnect the prop shaft.

No-one can tell if you'll have problems. I suggest, if there are no difficulties, you write it down to experience and tell yourself you're lucky to have got away with it. Keep everything crossed!

you never tow any vehicle with the wheels that are connected to the transmission on the ground. no fluid is being circulated through the transmission . all wheel drive should be on a flatbed, fwd front wheels up, rwd rear wheels up

You did not say how it was towed. If the front wheels were lifted then no damage at all. If it was towed with front wheels turning then a small risk is possible. I personally would not worry about it.

Hey Jerry, no, front wheels were not lifted, hope it won't cost me a big damage to the transmission. Thanks a lot

It was towed for 20 kilometers (about 13 miles) on a speed of 20 km/hr (13miles/hr)

I was told that this could hurt the gear box badly, is that really true?! And if yes why and what would happen exactly?

Thanks in advance..