how do i store some value to use after redirected from Payment gateway

not sure but when i integrated paypay they had a " if successful post payment url" you could set yourself and there was the means to get info. So it was only a matter of extracting that info , putting in a database, before the client sees something .like "payment successful" .

CMS CI4 A CMS system, runs out of the box written on top of CI4

Arch Book  CodeIgniter4 on Apache(pages 92-114) 

CI4-CMS Github