***Not a teacher***
As written, the two sentences do not mean the same thing.Venus said Gab had just used her then. Gab told her she could say THAT.
or should it be
Gab told her he probably did.
Do the two sentences mean the same thing?
'Gab told her she could say that', reads as if Gab is giving her permission to say it (which I don't think is what you are trying to say).
'Gab told her he probably did' is obviously Gab considering his previous behaviour and his assessment is that in all likelihood, he did use her.
Going back to the first sentence though, I think you may be reporting the following:
Venus: "You have just used me then"
Gab: "You could say that"
When saying "You could say that" in this kind of context, depending on the way it is said, it can mean a couple of different things. Firstly, it could mean "I think in the light of the evidence, it would be fair for you to make that statement". The second possible meaning is used quite often in speach and is ironic - "You could say that" can actually mean "That is certainly the case!"
For example, if you had been sold a car which then broke down a week later and cost a vast amount to fix, in a conversation with a friend, you might say:
Statement: "I don't think that car was a very good buy"
Response: "You could say that" (in other words "It certainly wasn't")
So I wonder if what you are trying to express in your first sentence is:
Venus said Gab had just used her then. Gab replied "You could say that".
This meaning is lost when reported using: "Gab said she could say that".