Zed-Avatar: Likely you have different regions selected for each store.
Here is the price on Steam by region:
https://steamdb.info/app/1091500/
Previously GOG offered Wallet funds to balance the cost increase (basically a Rebate) for such titles that were inconsistent, but this program ended I believe:
https://www.rockpapershotgun.com/2019/02/27/gog-ending-regional-pricing-rebates/
So if you are not in North America, the price may very well be different.
Edit: As an addendum, this is a relevant discussion about such pricing schemes:
https://www.gog.com/wishlist/site/ban_all_regional_pricing_from_gog_stick_with_your_principles I will quote, what I think is one of the most relevant comments:
"I don't normally post a comment on the internet but please don't [remove regional pricing]. I've been on both sides of the argument. I'm currently living in Australia where regional pricing actually means we will be paying more than American consumers. But I used to live in Thailand where regional pricing is great because 60 USD is about half of your monthly salary. So please don't there are gamers in many Asia and Eastern Europe who would never be able to buy games without regional pricing. I wish that countries that have to pay more than US get the same price as US after exchange rate but I understand if they won't." - Phant0m672