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