Unable to sign into father's Apple ID account

My father, who died in February of this year, had two Apple ID accounts, one for his comcast.net email address and one for his gmail address. His iPhone is currently insisting I try to sign into his gmail Apple ID account, and refusing to let me sign out without entering the password for that account (in order to turn off Find my iPhone). Therefore I am unable to even try to sign into his comcast.net account on his iPhone. All I can really do is try to sign into his gmail Apple ID account, which, when I try to do it with what I am pretty sure is the right password, informs me that this Apple ID is not valid. I tried to reset the password for his gmail Apple ID, but this did not work either: when I try to do so, it forces me to do it on his iPhone, and when I enter the new password twice in both password fields, and then try to tap "Next" to change the password, it generates an error message which reads "Cannot Verify Identity" and below that "Error Description not available".