The orientations of both Killua and Gon are uncertain at this time. Neither person has expressed explicit interest in anyone at all. As such, it is impossible to determine the orientation of Killua at this time. That said, many in the fan base ship Killua and Gon. As for whether or not Killua is in love with Gon, that is also uncertain. He undoubtedly loves his best friend but it is hard to gauge if it is more than that. I do think that there is the chance, for sure, that he cares about Gon romantically or could in the future. However, it is hard to know at this time. We would not likely find out until the end of the series when they are presumably much older.