Why did he say "v'Gam"?
Malbim: In addition to the evil of your action itself (making a Bris with them), harm will result. Due to this I will not expel them, and they will harm you b'medinos (politically) and in Emunah (you will stumble with their gods).
When did Hashem say so?
Rashi: I said now, amidst My anger.
What is the meaning of "l'Tzidim"?
Rashi: They are on your sides - legions and troops to plunder and take spoils.
Radak #1: The verse is abbreviated; it is as if it says liTzeninim l'Tzidim (like Bamidbar 33:55 - thorns in your sides).
Radak citing Targum Yonasan: It is as if the Dalet were a Reish, Tzarim (afflicters).
Radak #2: It is from the root Tzad (they will trap you), "Tzud Tzaduni" (Eichah 3:52). There is a Dagesh (dot) in the Dalet, in place of the missing middle letter of the root.
