This is one of those things that everybody has an answer for, but nobody has any evidence.
Almost nobody. The website Proof66 decided they were uniquely qualified to provide some actual... evidence. They did a controlled, blind-tasted experiment with 4 different ways of beating up the Gin. Check it out.
The bottom line on the question "Can you bruise a gin?":
Nope, it's complete bullshit. The only difference is a little in the presentation and the potential dilution of ice.
Now let's note two interesting pieces of context from our other answers here, as it may well be that it was once possible to bruise gin, but is no longer:
If, once upon a time, gin tended to have material amounts of oil in it, you're now talking about a different animal, in both chemistry and texture. It could have had very different characteristics than today's gins. The experiment I've cited doesn't address that.
If people were once in the habit of dipping highly-reactive metals into a drink (particularly one packed with volatile oils) in order to stir it, then yes, that certainly could have changed the flavor as well.
So, to say that 'bruising' of gin is a non-operative concept today is not to say that it was never an operative concept, particularly all the way back when the first Bond books were written, which was a very, very different era in terms of food & beverage production. But unless someone has other actual evidence - from either chemical theory or from controlled experimentation - it seems to be non-operative in today's context. (At least with gins made in the style of Tanqueray, which is admittedly a very commercial, very mass-produced product.)
Further, to say that 'bruising' is a non-operative concept is not to say that a vigorously-shaken martini can't taste rather different than one that is gently stirred. But again, this seems to have everything to do with ice-crystals and dilution, for which it's possible to control, and little to do with 'tossy' concepts like 'bruising'.