How would you reason with a JW that is okay to say Good Luck?
We can start by showing them what their own literature says.
(W 9-1-1980 QFR p30, par4) "Gamblers often magnify 'Lady Luck.' For them "luck" is not a word merely meaning good results, but is a superhuman influence like a god. Greed and pride move gamblers to trust in "luck."" (bold added)
It seems that saying 'luck' to simply mean good results is alright, but to rely on the God of Good Luck in serious ways, such as gambling, is off limits.
I don't know if Watchtower actually taught at one time that saying 'luck' in any fashion was bad because it was pagan, and later backed up on that, or if it was simply a myth that arose among Witnesses to begin with.
At any rate, when we say 'good luck' to someone, or 'wasn't I lucky because I found a dollar,' we simply mean good results.