OK, first of all, deciding to pick a frame size based on what someone else rides is absurd.

Second of all to understand frame sizing, you need to look at just 2 numbers - Reach and Stack. this is about TT bike sizing, but the concepts apply. No two manufacturers describe their bike sizes the same way, so looking at the number they use to describe the size is pretty meaningless.

(edit, clicked send too soon)

Thirdly, there's very little reason to suspect you will sit on your bike the same way Gilbert does - he's a pro, has been riding for years, and is likely fit to his bike to maximze aerodynamics at the expense of comfort. Unlikely that you will want to ride the same position, even in the unlikely event that all your body dimensions and flexibility are identical.

Good luck with your search - getting professional fitting help is well worth whatever you spend on it.