Do you think Killua would get jealous if Gon got a new best bud/traveling partner while they were separated?

never thought of that before, but yes, absolutely. all the previous relationships killua had before meeting gon were built on the sense of exclusivity, of being the other’s ‘one and only’. no matter how toxic and abusive those relationships were, they set a certain pattern that killua follows simply because he is still a child that doesn’t know any better.
that said, i do think that gon is also jealous of alluka for being killua’s new ‘number one’. gon is used to being the center of attention, to things going his way just because he insisted on it, to always having people follow him around and chase after him (just like ging, thanks togashi). being left behind for someone else must have been quite a blow to his ego.