grav/cold dominion is it worth building

will i be able to solo? Farm (even if slowly)

Solo? Yes. Everything can solo at some setting. Farm? Probably not. Farming generally means defeating a large number of weak foes quickly. Gravity provides stronger single target damage and only average AoE damage.

Any particular reason for pairing with cold? Storm shares quite a few powers* with it and has some tricks that work well with Gravity, for example stacking stuns with Thunder Clap or Wormholing mobs into a corner where you pin them with Hurricane. If you intend to solo a great deal I would favor Grav/Storm, but on teams Cold Domination is an excellent buff set.

*Snow Storm is in both sets. Arctic Fog is essentially Steamy Mists. Sleet is the equivalent of Freezing Rain.