How did Avimelech answer Avraham's accusation?
Seforno: Even now that Avraham had informed him about the theft, he argued that he could not imagine who the culprit could be, because had one of his servants been under suspicion of doing such a thing, he would never have retained him.
Why did Avimelech add "ve'Gam Atah Lo Higadta Li"?
The Brisker Rav: Avimelech turned to Pichol and said 'You didn't tell me either'.
Why did Avimelech add the last two phrases - "ve'Gam Atah ... ve'Gam Anochi ... " - in the Pasuk?
Seforno: In response to Avraham's claim that, as king, he ought to have taken action, Avimelech replied that a king would take action either following a complaint from the person who was robbed or because he heard about it from circulating rumors, neither of which was the case here.
Rosh: Avraham swore in the name of the One who will later say "Anochi Hashem." Avimelech replied that he had never heard of such a god. Refer to 21:23:2