What was Hashem's response?
Rashi and Seforno (Pasuk 7): Hashem responded by instructing Moshe to pass on every message to once, as he heard it from Hashem, and that Aharon would elaborate and explain it to Par'oh in more detail.
Ramban: 1 He informed Moshe that, from now on, He would come before Par'oh together with Aharon, where, in his new capacity as Par'oh's god, he would quietly inform Aharon what Hashem had told him, and Aharon, as his spokesman, would pass on 2 the message to Par'oh. 3
Targum Yonasan: He told Moshe that there was no reason to be afraid of Par'oh, since from now on, Par'oh would fear him like a god, and Aharon would be his prophet.
See Ramban, 6:14.
In the same way as Hashem gives information to His prophets, who then pass it on to the people and rebuke them (Ramban).
This highlighted the greatness of Moshe, who, in his great humility, was embarrassed to appear before Par'oh with uncircumcised lips, and who now appeared before him in the form of a god (defect intact) - Midah Keneged Midah. Indeed, the Pasuk (in Bo, 11:3) will later write about him that he was exceedingly great in the land of Egypt (Ramban).
What does it mean "I made you Elokim to Pharaoh"?
Shemos Rabah (Sha'anan) 7:7, Tanchuma 9, Mechilta d'R. Yishmael Masechta d'Pischa 1, Yalkut Shimoni 179: You are a judge to him. 1
Hadar Zekenim, Da'as Zekenim: You are like G-d to him, because Pharaoh made himself a god, and said "the river is mine, and I made it." 2
Hadar Zekenim (12:12): 'elohim' here means a great person, like "uv'Chol Elohei Mitzrayim E'eseh Shefatim." 3