Can the author of software make it proprietary after making it gpl?
Yes, the author (or whoever holds the copyright) can do as they please, changing the licence at will. But anyone who already received a copy under a free licence continues to enjoy the rights that accompanied it; the author cannot take those away merely by ceasing to distribute further copies under the same terms.
Case in point is SSH: It was open source up to version 1, version 2 (clearly a development on version 1) is closed. OpenSSH took version 1 (still open source) and created an extension handling the new protocol, released as open source.