I have walked it . . . barefoot most of the way . . . so I am something of an authority on the surfaces :-)
They vary wildly from suburban style paths to squishy mountain turf, taking in loose shale rocks, beaches, and the shore of Loch Lomond along the way. Do not even think about the shore of Loch Lomond. That would be most of the day carrying your bike over continuous boulder-fields and ankle-breaking tree roots. Riding along the bottom of the lake with a snorkel would actually be easier. There are also some very awkward crawls under railway lines etc further towards the north.
Also, given the soft, squishy and wild nature of some of the terrain, I would be embarrassed at the amount of erosion a bike would cause.
Check out the WHW website for advice. Bits of it certainly are cycle able, but personally I would feel a bit anti-social cycling the whole thing. As I was ascending the Devil's staircase out of Glencoe a couple of mountain bikers came through. Looking down the mountain, I could clearly see that their passage meant that about fifty people strung out over two miles or so had to step off the narrow track to let them through. This is one of the most popular walking routes in the world, walkers come from all over the planet to do it, and 'traffic' can be quite heavy.