I don't understand the exclusives. There is

1 wallpaper/ soundtrqck and stuff
2 inside game
3 tengu mask

I understand the inside game promo but why do i need to fill in a form for the tengu mask? Is the installer on another website? The wallpaper/soundtradk are these going to be free for people who buy post releqse? If anyone could clarify id appreciate it.

The publisher wants to harvest your email and send you ads. No, they won't be free and specifically the wallpaper and masks probably won't be available at all.

To clarify, there are three packs of incentives:
Inside: GOG's promo deal
soundtrack + wallpaper + sack + teapot: publisher preorder exclusives
tengu: "free" publisher dlc in exchange for newsletter signup

Is the soundtrack and walpaper goodies available on gog? I couldnt see it listed on the store page's goodies section. Normally goodies are listed but this isnt

They're not currently downloadable from the preorder stub in the library -- there's only the countdown - but most likely yes. I don't remember a single bonus item GOG advertized that wasn't actually available through GOG.

The tengu mask will likely be delivered through GOG via a bonus code for the secret unlisted DLC, that's why they want people to choose a platform when signing up. (Because what's the alternative, providing a downloadable file to stick it into a game folder? In the age when many people don't know what a file system is?)