Need to update payment gateway on another page of the website. Also fix some functionality on core php. project

We have a project built upon core PHP, We have integrated 2nd payment gateway on our website. , but we need to update this gateway on another product page too, where our previous gateway was working fine, now we need to connect this new gateway there. Also, some functionality is not working on the website due to the new gateway, like thank you page. We need to fix it.