Gibson Greetings Inc., the nation's third largest greeting card company, sold out its initial public offering of 3.5 million shares at $25 1/4 bid per share yesterday through an underwriting syndicate managed by Lehman Brothers Kuhn Loeb Inc., a Lehman Brothers official said.

"The offering sold out and the syndicate has been disbanded," said the official.

The Gibson offering is a potential bonzanza for former Treasury secretary William Simon, who holds just under 2.4 million shares, or 24.6 percent of the company.

Simon is part of a group that purchased Gibson from RCA's C.I.T. subsidiary in 1982 for $58 million, plus the payment of another $22.5 million in loans owed by Gibson to C.I.T.

Gibson designs, produces and markets greeting cards, gift wrapping paper and related specialty products.

The sale is part of the expected offering of about 10 million Gibson shares over the next 18 months.

If the entire 10 million shares sells in the $25 to $26 range, Simon's group can expect a profit of roughly $180 million.

Proceeds to the company will be used to provide working capital, reducing Gibson's need for short-term borrowings.