1)
What would cause the Goyim to say so?
1.
Radak: if You will not do Chesed with us, and You will leave us among the Goyim.
2)
Why would they say "where is their G-d"?
1.
Radak: They would say, now that we rule over them, where is their G-d? He should rise and help them!
2.
Malbim: It was already publicized among them that You are Elokei Yisrael, and do miracles for them.
