When/how did Greenberry tell Kelly he was switching? Did he lie about it?

Greenberry never told ND anything. According to Kelly, ND didn't know about the switch until Greenberry's HS coach told them this morning that Greenberry signed with Houston.

Allegedly ND spoke to Greenberry on Tuesday night and he didn't mention anything to them about being unsure, so they went to bed thinking that he would sign with ND on Wed.

And, did Gunner do the same thing? or did Gunner just change his mind?

I really don't know about Kiel's situation w/r/t when he spoke to LSU. One difference is that Kiel defected 3 weeks before NLOID and LSU was able to recover with Jeremy Liggins. Greenberry blindsided ND the morning that letters were being sent into fax machines w/o warning and without time to line up a replacement.

I don't mind DG changing his mind, provided he at least informed the ND coaches he was switching.

It doesn't appear that he did. He tweeted something yesterday to the effect of "shocking the world", so he obviously knew that he wasn't going to ND yesterday when he spoke to people at ND and said he was solid. Then when the time came to inform ND he had his HS coach do it.

If he did that, and Kelly still called him out, then Kelly acted like a douche.

I don't know how I'd handle it but if I were Kelly I'd be damn well pissed over how Greenberry handled the situation. If DG wanted to go to Houston, fine. But it appears that he took it all for a game and wanted today to be about him, so he pulled this surprise on NLOID and screwed ND in the process. If he had told Kelly that he was having questions or seconds thoughts as recently as last Fri then ND could have brought in a recruit or two just in case. As it was Greenberry told ND he was solid, ND cancelled some planned trips by WR prospects, then Greenberry "shocks the world" and gets his 15 minutes, and ND is left standing there with a giant hole in their class.