Robinhood blocked purchases of GME last week, only allowing sales. CashApp is owned by SquareCash, whose CEO, Jack Dorsey, heads both Square and Twitter.
Square is, as of Monday morning, still preventing the purchasing of GME stock through their app, although other stocks are available for purchase.
CashApp does have a support document mentioning the limiting of sales for AMC, but has no such document for GME. That seems suspect.
If you had the stock, they would allow you to sell it, but to not allow you to purchase it is the same kind of alleged market manipulation Robinhood app engaged in.