I bought Gulden tokens at the peak, and I do not know what to do now, whether to sell or not? Can he grow up again?

Just sit it out, wait for the update
And they plan some sort of an update? I just did not hear about it, now I'll wait for it, maybe then the price will go up.
Yes it was expected "soon" in May 2017.... Now I think we are at "very soon" so I suppose the release will be done in December 2018 , or may be in 2019 , or why not in 2020 ... if the coin is not dead before
Rijks brother has done 2 iOS updates in 4 months, why can't he help the other devs finish PoW2 and SegSig?