Loss of an unlawful gain wouldn't be an actionable harm in any case.
"I intended to buy at a higher price in order to manipulate the price of an asset well above its value, and was harmed by being delayed until a later time where I could buy it at a lower price" -- not going to fly.
If RH is inadequately disclosing the risks of margin trading then some of their customers that had positions closed due to margin requirements might have cases.
The vast majority of GME buyers could never be considered guilty of unlawful market manipulation. "I intended to buy the stock because I thought it would go up" is a pretty good reason. (OTOH I don't really believe such investors have a strong case against RH).
"I was harmed because I was delayed from buying at a higher price and instead could only buy at a lower price later!" -- only being able to buy at a lower price later is only a "harm" in the sense that it prevented price manipulation.
"I intended to buy at a higher price in order to manipulate the price of an asset well above its value, and was harmed by being delayed until a later time where I could buy it at a lower price" -- not going to fly.
If RH is inadequately disclosing the risks of margin trading then some of their customers that had positions closed due to margin requirements might have cases.