Sens will be looking for a lot more than this. Especially after the Duchene trade.
Mersch already cleared waivers this season so if they wanted him they would have taken him then. IMO, Andreoff is also a type that would clear and I don't see fitting into the Sens lineup. Gravel hasn't been effective in Ontario really, let alone in NHL.
So its essentially 2 #2 picks, Martinez and Clifford for Karlsson.
I see them asking for at least two #1 picks. The Kings would likely have to move Martinez or Muzzin elsewhere for a #1, and include that in a deal to the Sens.
Imagine Doughty and Karlsson though.