Worth investing against blissy?

No. He would be good against bug, fighting, or poison; but you seldom see those types in gyms.

I have a 97.8% Gengar with the same moveset at CP 1492, that I use for prestiging against Blissey. It can't finish the job on its own, but it dishes out enough damage before it faints that I can bring in another attacker such as a Machoke to finish off the Blissey.

Maybe consider keeping it at its current level as a prestiger for a while, until you get enough candies to power it all the way up in one shot. For me, a Gengar in the 1400's is useful, and a Gengar in the 2200's is useful, but one in between doesn't really do me much good.

Gengar does insane damage with ideal moveset but HP is just too low to make him viable for daily use.

In fact Dragonite has more HP than Gengar and people think Dragonite is easy to defeat.

Blissey resists ghost moves so I'd not power it up just for Blissey until a more viable gym defender is there or a gym rework to make ghost moves effective against a common high CP Psychic gym defender (maybe Mewtwo event?).

Unlike Eggs, you also probably don't want to just spam attacks when attacking Blissey due to SE Zen Headbutts and even STAB pound would probably hurt.

1461 is a little high for most gyms to prestige, Gengar isn't the best prestiger due to high CP, low HP and incorrect typing for the common gym defenders.