GAF Corp. said a management group led by its Chairman Samuel J. Heyman has lowered its $2.3-billion buyout offer for the chemical company because of market conditions. The new offer, which includes cash and notes, was difficult to value because the debt portion of the offer was not fully disclosed, arbitragers said. However, one estimated the bid could be about $1 billion lower than the original offer. Arbitragers profit by trading on takeover stocks. The value of the offer was also complicated because the number of GAF shares outstanding has dropped to 27.7 million from the 33.5 million outstanding on Sept. 8, when the original bid was made. GAF initiated a stock buyback plan following the initial offer. Heyman had said in late October that he was reconsidering his offer to buy the Wayne, N.J.-based company because of rising loan costs.