I bought a GOG bundle on Humble Bundle. I appreciate them offering GOG bundles because it's my preferred game vendor. I assumed that we would be getting keys for the games but it turns out that we need to "link" our humble bundle account to the GOG one in order to claim the games as per the instructions here:

https://support.humblebundle.com/hc/en-us/articles/360047622913

My question is what information is being shared with Humble Bundle if I link my account?

If it's anything like the steam CONNECT linking, it's basically whatever you put on for a public profile i think for the most part. It could be as little as just your username (and by extension, probably a shared email address); Though on Steam you had to show what games you had, which was more to justify getting the free copy on GoG; Not activating a new purchase.

The obviously don't want people reselling the keys which is why they do it but then they should just give a single key for all of the games. By forcing the link, you are limiting the games to a single person so there is no reason not to claim then all in one go. I am just not a fan of links.

Reselling the key shouldn't matter, since it should be about the same value should it be Steam, GoG or HB. Unless it's some bulk deal for a very reduced price.

I don't know. It will just have to be personal preference. If you only have 1 HB and 1 Steam and 1 GoG account, connecting them is fairly benign. Especially as you won't technically even own them anyways.

Likely it is about statistics and records. And perhaps GOG enforces it.

That said, very clearly you link to GOG once and then every claim is automatic after that, as opposed to going to GOG and using a Redeem code for each game. So it is an automatic service they offer, albeit with no opt out or work-around.