And what if I bought your asset on this site (https://www.gamedevmarket.net/asset/complete-gui-essential-pack) because I had the same problem, that I didn't have PayPal, and could only pay through a bank card. If you update the asset in the future and add new content to it, will the updated version be available for download from this site or will I have to buy it again?
Yes, I will update the new version as well on the GameDevMarket same as in Itch.io and I think you won't need to pay again for future updates, you can redownload again on that site to get the latest version. But keep in mind, whenever I update my pack to the newer version on GameDevMarket will put on hold on my request for a review, so therefore the update won't be the same day published as in itch.io (usually takes a day or several days).
Thank you for bringing this up, In my next update I will try to help share this product link (from GDM) to here (Itch.io) for those who don't have Paypal.