Thank you, this was super helpful. I also used the FTimespan::ToString(format) method like this:
float GameTime = UGameplayStatics::GetRealTimeSeconds(GetWorld());
FTimespan TimeSpan = FTimespan(0, 0, GameTime);
return TimeSpan.ToString(TEXT("%m:%s"));
Just leaving it here as an additional note because it took me a while to figure out how to use the formatting options. Basically what I was missing is that the expected TCHAR* is generated by the TEXT macro.