Is it kufur to give a gift card to a non Muslim which you think might use it for Haram purposes or mabey even kufur. Such as they buy a painting canvas to paint their god but you didn't know what they are gonna do

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu`alayhi wa sallam, is His slave and Messenger.

First of all, we believe that you, dear questioner, have much asked about topics related to disbelief, apostasy, and conversion from Islam, which clearly indicates that you greatly suffer from whispers on these topics. So, we advise you to fear Allah, the Exalted, and to keep away from these whispers, so that you would not fall into their traps or be afflicted with bad consequences.

As for the original ruling on giving presents to the non-belligerent non-Muslims, is permissibility, which is evidenced by the act of the companions who used to give presents to their relative polytheists. ʿUmar ibn Al-Khattab’s action is a case in point. So, it is permissible for the Muslim to give presents, including money or a voucher by means of which the person could buy something, to the non-Muslim unless he (i.e., the Muslim) certainly knows that he would buy unlawful matters with this money – a point at which the non-Muslim should not be supported.

Allah knows best.