Forgiving the Gillian thing, which was pretty dumb to do, it is tampering with the timeline
Remember, though, that wasn't Kirk's idea. Gillian was the one who grabbed Kirk as the transport beam took hold.
I suppose Kirk could have refused to let her go with them but if she is determined to stay, then there's not much they can do.
The Guardian never stated that you can't bring someone from the past into the future.
Perhaps not. But there simply was no oppportunity to bring Edith back.
If you'll recall, Kirk and Spock had to wait until they encountered McCoy again before they could return home. This only happens right before Edith is hit by the car.