This is actually a very common criticism that I come across. It is feasible that prices will decline as the economy grows under a gold standard, and this they claim will incentivise stashing cash under the matress (where it will get a definite return) rather than risking it.
The key thing they don’t recognise is that entrepreneurs have a low time-preference. They forego present goods for future goods, ie. they don’t consume, they invest.
So a higher purchasing power of money tomorrow may intice them into investing money today.
A bit more: https://forum.freecapitalists.org/t/did-i-get-this-right-about-deflation/4021/14
Also, point out that “growth deflation” under a gold stsndard did not put an end to entrepreneurship in the US between 1870 and 1896. In fact, there was steady growth throughout the period.