In most cases you can do what you are proposing. However be warned that by disregarding the authentication you clearly loose message authentication and bit flipping in AES-CTR encrypted stream is trivial.
You can do what you are proposing if the AES-GCM IV size is of 96 bits. AES-GCM supports also longer sizes for IVs and for those cases you would need GHASH to find the correct IV used by CTR. That having being said I believe 99% of implementation supports only 96 bit IV for AES-GCM (and rightfully so).
To obtain the CTR IV, just append the value 2 encoded as 32-bit big endian integer to the 96 bit GCM IV (equivalently adds in C notation "0x00,0x00,0x00,0x01").
Edited to change from 1 to 2 of the initial counter value. GCM acutally defines the starting counter as starting with 1, but increments it before the first encryption, thus resulting in a 2. Thanks to @TrickyDixon for pointing this out.