Keep in mind that "gravitational accretion" simply kicks the can a bit further: the chunks that accrete gravitationally must themselves form and congeal somehow.

What I understand of solar system formation is that all "metallic" solar systems (though with a substantial portion of elements other than hydrogen) from from the remains of earlier stars. So you have a nova, supernova, stellar collapse, collision (stars, white dwarves, neutron stars, ...). This ejects heavy materials (principally H, He, C, O, N, though others --- water is probably the most common non-elemental molecule in the Universe), and also creates pressure waves and imparts angular momentum. Both factors can draw material together. There's also probably a lot of plasma in the mix, so that different portions of the cloud carry different charges. These may be attracted, repelled, and spark discharges (which might themselves melt and fuse material). There may also be surface-tension effects and other factors at play. Somehow, clumps form. Gravity is a force at work, but not the only force.

Disclaimer: not an astronomer, just interested in far too many divers subjects. If anyone has anything authoritative to say on this I'm all ears. Most references I can find are either extremely basic, or address the formation of asteroid belts but not the objects within them.

There's some hypothesis on formation (condensation, shock waves, jet flows) here: http://www.psrd.hawaii.edu/Sept02/isotopicAges.html