Generic steps to find what you need to change in a GYP:
1. Create your GYP file
2. Run GYP on it
3. Open the Xcode project file in Xcode
4. Change the thing you need to change in it (your case, code signing)
5. Close Xcode, duplicate the file you just modified and rename it as a "copy"
6. Run GYP on your original again.
7. Run a diff against the outputted clean version vs the modified copy (I suggest the Xcode-provided "FileMerge" tool), and you'll find what you need to add, typically in the 'xcode_settings' block.
Cheat sheet for your situation, you can set the code signing stuff specifically like this:
'xcode_settings': {
.
.
'CODE_SIGN_IDENTITY': 'iPhone Developer',
'PROVISIONING_PROFILE': '',
.
.
}
To find what values you need to put in both of those key / value pairs, run the procedure I stated above.
iPhone Developer and {blank} should work in most cases, but are probably not the values you want in your case.
Good luck.