I've tried everything to plant the wire tap. Sneaking it onto him (and desk), dropping it from my inventory, putting it/sneaking it on his guard. I've failed the speech with him, so this is my only option for finishing the quest. This is for the 'getting Gizmo' quest in fallout 1.

You have to 'reverse pickpocket' him to plant the bug. Just use steal on him, but instead of taking something from his inventory give him the bug.
If you keep getting caught then just leave him for the time being and go level a bit and increase your 'steal' skill.

I also believe that you can leave it on a bookshelf in his office...I think I did that once becasue he kept catching me trying to leave it on him.

However it's been so long since I've played Fallout 1 I can't say for sure if it worked.

Altra

Thank you. I'm going to try that.

If you've got a reasonable speech skill, you can just skip that part. Just Talk to Gizmo about Killian Hypothetically getting killed instead. (the tape recorder is a wire)

This, all I did was this, and then I went back to Killian, and he had the info he wanted.