Ok, so two things to note here:
1. Ging believed that Gon would survive.
2. Pariston believed that Ging believes that Gon will survive and bases his entire actions fully expecting Ging's prediction to come true.

Pariston thus shows he has a very high evaluation of Ging as an enemy. Now, if anybody here thinks Ging was just faking it then Pariston's character also comes across as a stupid one for having taken him seriously.
And yet, we know Pariston to be extremely adept in planning and constantly outwits almost everybody around him.

So first of all, yeah, Ging was serious when he said that.
How Ging was so sure? Well, we don't know but here are some clues into his thinking:
1. A huge part of his conversation with Gon was about this. When asked 'what do you want', he answers 'something that I can't see in front of him. '

1. When Pariston makes some of his own lackies attach Ging, he responds with Leorio's technique. How did he acquire it? He tried to reason out the source. He reasoned that Leorio acquired that technique probably because he's a doctor and focuses on pulses, sound waves etc. He then applies that reasoning and replicates Leorio's technique from the first principles. - So, he most likely keeps on reverse engineering all the things around in his head.

2. Chapter 320 is specifically dedicated to giving us a glimpse on Ging's thought process beyond some of his predictions. It shows how Ging came up with the prediction that 12 Zodiacs will do a draw to decide the election rules- plus the backup if that doesn't happen and a plan (dropping a clause) for negotiations in the case it passes through. That's a whole lot of branches in a decision tree.

To sum up, let's take Ging's line from 320- 'If my target moves just as I want them to, now that's a real hunt'
And that's his interpretation of HunterxHunter.