The thing is the invasion of Gilneas was a Horde invasion, not Forsaken. They were the vast majority of the forces for the obvious reason of location. However, Sylvanas didn't want Gilneas, Garrosh did.
The only reason Gilneas has any value to any non-Gilnean is its proximity to the Alliance held southern Eastern Kingdoms.

Garrosh wants it, the Alliance doesn't want Garrosh to have it. Same deal with Tol Barad.

Besides the 'forced conversion' happened to three Gilneans, who all were killed in Shadowfang Keep. The rest became Worgen, were killed, or escaped.

@Plainswander
She used to not care at all about the Forsaken, and use them like water. Same with her rangers while living.
Now she wants to protect them, because they protect her.
At least that is what I got from her perspective on them going from 'Arrows in her quiver' to 'a Bulwark against the infinite'.

You shoot an arrow and don't care about it as long as it hits your enemy, a shield has to be maintained, cared for, or it will fail you.
Again, that's just what I got from it.