Evening p&l!
I rode Gilpin's Gallop as my first audax a couple of years ago - among the bikes parked outside were skinny-wheeled full carbon Colnagos, so I don't think your 20mm tyres will be an issue. However, for comfort you might prefer to fit something a bit wider. It's all about personal preference, comfort and what you're used to - I'll have 23mm tyres on mine.
I'd recommend carrying the same tools you'd usually have on a club run, i.e. at least enough to fix a puncture. I'll probably take my Carradice, but I rode it last year with just a wee saddlepack.
I'd recommend a wee cafe lock, for fastening your bike outside the controls in Boroughbridge and especially Middleton-St-George. You might be OK leaving it, but there'll be plenty of non-cycling people about, so I wouldn't take the chance myself.
For food, you may need nowt along the way, as this year there'll be a feed stop at Girsby with bacon sarnies and cake, as well as the Morrison's at Boroughbridge. Most people carry an emergency something, e.g. a gel or flapjack.
No mudguards - no problem, really. People will probably take the piss if it rains
I'll be on a red (fixed) Dave Yates. See you there.
ETA: Gah. MSeries has beaten me to most of this.