For the purpose of discussion (hence why I'm posting this as a comment and not an answer):
- What is the rest of your team currently like? Thunderbolt is a pretty prized TM, and while Gengar could use it quite well with that 130 Special, perhaps another team member would get more mileage out of it for coverage purposes?
- Would you consider other moves instead? Say, Night Shade for guaranteed chip damage, or Double Team to set up on a particular trainer's 1st Pokemon and be nigh unkillable? (Guess it's prob to late for Night Shade, no move tutor in Gen 1 ...)