Gravity forms using as new post form for different categories

I have two gravity forms that allow my user to post me threds from site, but i want their posts be displayed as new posts of a specific category.

I have found this link below and yet try it and it returm me php script error: https://www.gravityhelp.com/documentation/article/gform_after_submission/

Let’s someone who have yet do it and successed tell me in simple words step by step, what to do: files i have to modifie, what i have to write….

thanks