all techs are given a priority to be "bulbed" by each GP. you can find thoses lists on the wiki, at each GP article ....
when you want to lightbulb a GP, it looks "oh, I'm a GS, what is the reachable tech that has the highest priority for me?"
as to how the priorities where given / chosen ? no idea. for flavour, balance and who knows what. maybe some people from the mod team knows the reason behind each choice and value for each tech and each GP but I'm not sure this hypothetical personne exist.
when you want to lightbulb a GP, it looks "oh, I'm a GS, what is the reachable tech that has the highest priority for me?"
as to how the priorities where given / chosen ? no idea. for flavour, balance and who knows what. maybe some people from the mod team knows the reason behind each choice and value for each tech and each GP but I'm not sure this hypothetical personne exist.