The kiss could change Gohan's view of 18 completely. He may start to see her not just as an ally, but as someone he truly cares for deeply. This could lead to him being more protective of her. 18, in turn, might start to open up more about her past and her feelings, as she realizes that Gohan is someone she can trust with her secrets. It's a big step in character development for both of them.
It could have a profound impact on their characters. Gohan, who has always been focused on his duties as a protector, might now find a new motivation in his relationship with 18. He might become more relaxed in some ways, yet more determined to keep her safe. 18 could become more empathetic. Since Gohan is a very empathetic character, his influence through the kiss and the relationship could make 18 more in tune with the feelings of others around her.
For Gohan, the kiss might make him more confident in his own emotions. He's always been a bit reserved when it comes to relationships. This could be a step forward in his personal growth, making him more assertive. For 18, it could soften her tough exterior. She's a strong fighter, but the kiss might show a more vulnerable side of her.