You can usually tell who is a tourist and who is a Gibraltar resident from the way they talk with the exception of British people who've recently moved to Gibraltar.
Anyone who lives in an area that gets any day-trippers/tourists will tell you that you don't need to hear them speaking a foreign language to know they are tourists - it's not unique to Gibraltar! Residents can spot a tripper at a hundred paces!
In my experience there is nothing unusual about buses being full of tourists and trippers - you see them all the time everywhere they go, both here in the UK and abroad. From my own personal experience (having lived in a few tourist hot-spots) I would say that it's just us Brits who are retiscent about using the buses when in areas beyond our 'comfort zone' - whether that be in the UK or beyond.
Of course much of that retisence whilst abroad is down to the language difference. Only we (and the Americans) would be so arrogant as to go abroad and expect everything to be in English for us. If we don't know how to ask for directions or help (and I count myself in that category I'm ashamed to admit) then we are far less likely to risk it.
But of course that doesn't apply here in the UK, and yet you are far more likley to see a service bus full of foreign visitors than you are to see a bus full of British visitors from elsewhere in the UK - the specific tourist routes and services notwithstanding. One can only speculate as to why that is.
And before anyone says 'but how can you tell they are tourists if they are British' let me just repeat - you can spot 'em a mile off! You can tell by what they are wearing and carrying, their behaviour, how they walk and what they look at. Admittedly not all British trippers on a Jolly Boy's readily identify themselves in this way, but most do!