I think Giles would ride a Ducati.
Not il Mostro, either. I see the man on a 748. Yellow. Dry clutch rattle, desmo valve drone, and all. I don't think he would go all out on the 996 monoposto, though that might be the single sexiest bike Ducati ever built.
But Brit pride! I hear you cry. Okay, a Speed Triple. I see him going streetfighter if he chooses the sweet bikes of his mother country. The cafe racer thing would remind him of the bikes he stole in his youth.
I will now expose my reasoning.
Giles likes funky. Evidence: the Citroen.
Giles has European tastes. Evidence: the 2-door tramp.
Giles was having a mid-life crisis. Evidence: 4th season.
Giles is secretly cool. Evidence: need I write that essay for you, dear reader?
I will now answer all your objections.
Why not a Beemer, the bike of wonks from all nations? Because: not thrilling, too damn nerdy, best suited for long tours, which Giles isn't going to take.
Why not a cruiser, some under-engined American thing weighted down with an extra hundred pounds of chrome? See above statement about Giles being cool. Also, European. Also note that he's in California on the coast. California is sportbike heaven. All he has to do is point the thing uphill from town and he's in the twisties. And I will testify to you that heading north from where he is on highway 1 is also fun. Though you have to be willing to twist the wrist and pass the motorhomes when you get the chance.
Okay, you've made the sportbike argument. Why not the Daytona, then? Well, it's just not sexy, somehow, the way the Speed Triple is sexy. Or the Duck.
Why not a J-bike, something like the Honda CBR? Very practical, scary fast in any displacement, sweet to ride, yes, but not funky. The sound of an inline four is the sound of an angry bee. Contrast with the sexy grunt of the twin on the Duck, or the distinctive sound of the triple.
You may consider this entry to be my attempt to wash out of my mouth the taste of fics that put him on a "sleek black motorcycle". Specificity, people, specificity.