I need help with casting using C++

Try:

YOURGAMEMODECLASS* gamemode = Cast<YOURGAMEMODECLASS>(GetWorld()->GetAuthGameMode());
if (gamemode)
{
	...
}

or:

#include "Kismet/GameplayStatics.h"
     
YOURGAMEMODECLASS* gamemode = Cast<YOURGAMEMODECLASS>(UGameplayStatics::GetGameMode(GetWorld()));
 if (gamemode)
 {
     ...
 }