For the record I've recently done one X26 transit LGW-LHR T5 and it was a convenient option, taken that the buses on the route X26 have a luggage rack on the lower deck. Traffic is perhaps the only real variable in this option, if you've timed yourself otherwise properly for a bus departure and don't have to wait a lot. I personally only travelled with my carry-on, but a suitcase would've been easy to carry with me as well. Easy to change at Hatton Cross for the freeflow Heathrow transit with Piccadilly line. Although I recommend doing the Central Bus Station, because you get to enjoy nice RWY views on the upper deck.
£4.70 is the current off-peak fare for X26 and Southern combined.
It's not on record in this thread or in the FAQ, but the cheapest all rail option at the time being is Piccadilly line: LHR T3-Earl's Court (or switch at Barons Court to save some walking), District line: Earl's Court-Wimbledon, Tramlink: Wimbledon-East Croydon, Southern: East Croydon-Gatwick. In total £6.20 off-peak. And yes, a tram is rail traffic, although not railway traffic.
I was planning to try a Feltham transit this week for comparison, but due to the SWR strike, I rather skip it now, as the reduced service is just every half an hour.