Except Plagueis was pretty much invisible as well. Maybe not TRUE invisibility, but he was able to sit around on Coruscant without even Master Yoda knowing he was there. He would have to literally walk into the Jedi Temple and say "Hey I'm a Sith Lord here to kill you!"
If Yoda and the entire galaxy couldn't see him, what makes you think G0-T0 can?
Plagueis isn't really a hands-on kinda guy. If he has the tools to take G0-T0 out, he will use them.
GO TO is a droid and not effected by force mind tricks like organics are. He can also take over droids, electronics and ships. If he was in our world he would be that evil thing in the internet.