I have a bunch of old tech reports I copied when a grad student written by other people. I want to throw them out but would rather scan them into ResearchGate first. For example, INRIA tech reports on various unification methods. Is this OK or should only an author of a report scan it in? I'm not claiming it as my own, I would tag it to the authors. I just think they are a shame to throw away completely.
No, it's not OK. You are not legally allowed to distribute these reports if you don't hold the copyright and haven't been granted the right to distribute by the copyright holder.
To add a non-legal reason: You don't know if the authors would like to see these reports on ResearchGate. They might be embarrassed by what they wrote in them, they might want to update them, they might want to publish them as a journal article, they might boycott ResearchGate, ... It would not be polite to upload them without their permission.