The most valuable gas giants are, oddly enough, not the rarest or hardest to find ones. The most valuable gas giants are Class II. Class II are fairly easy to pick from the system map, as they are usually very pale, pastel-coloured compared to other gas giants. A Class II is worth about as much as a HMC, so they are about as worthwhile going out of your way to find.
Personally, I'll consider investigating any Class II less than 50,000 Ls from the jump-in point, more than that if there are other interesting targets at the destination. Other gas giants I won't bother with unless I happen to come into range of one while investigating something else.