The bus is best i think unless you know in advance of problems on the M25, as other have said this is key. If not Tube then Gatwick Express takes about 1 1/2 hours from T5. If your going direct to gatwick (ie; not stopping in central london for any time) then the Heathrow Express is not worth it. Because you need to go to Victoria to get the GEX the tube is just as fast and cheaper.