***Not a teacher***
...is fine.Venus said Gab just used her then.
...doesn't sound right.Venus said so Gab just used her then.
Alternatively, you could use "Venus said that Gab just used her then" which is (in my opinion) almost interchangeable with "Venus said Gab just used her then."
Using 'said so' is used in the context of referring back to something that was said earler
"I am allowed to park here, the policeman said so."
(i.e. the policeman told me I was allowed to park here and I am now reporting that to someone else).
You may hear parents saying
"Because I said so!", when their child is asking why they have to do a particular thing.
Student or Learner