Yes, Breloom's Dynamic Punch will definitely hit Gengar, but it won't matter because Gengar is immune to Dynamic Punch anyway. Additionally, Gengar will hit Breloom with all of its moves, regardless of the move's original accuracy.
Telekinesis is not a good move competitively, considering the turn you use the move can be used for something more useful like Substitute or just attacking. Usually, the most common attacks don't warrant Telekinesis since they have high accuracy anyway, with the exception of things like Focus Blast/ Stone Edge etc.
During the effect, the target is immune to Ground-type attacks and the effects of Spikes, Toxic Spikes, and the ability Arena Trap as long as it remains active.
If the target uses Baton Pass, the replacement will gain the effect.
Ingrain, Smack Down, and Iron Ball override this move if the target is under any of their effects.
Fails if the target is already under this effect or the effects of Ingrain or Smack Down.
Pokemon protected by Magic Coat or the ability Magic Bounce are unaffected and instead use this move themselves. This move cannot be used while Gravity is in effect.
Note that it's the target that's immune to ground moves/ spikes/ toxic spikes/ Arena Trap, not the user.
Smogon - Telekinesis