Gateway Computer Systems, a Huntington Beach computer store chain that filed for bankruptcy protection last week, has tentatively agreed to be purchased by a San Francisco computer retailer. In its filing for protection from creditors under Chapter 11 of the U.S. Bankruptcy Code, Gateway listed assets of $23.2 million and liabilities of $22.6 million as of Jan. 31. However, an attorney for Gateway said the company's financial condition had deteriorated since then with a series of losses. Martin Brill of the Los Angeles law firm of Robinson, Wolas & Diamant said Gateway had agreed to be purchased by Boardroom Business Products in a stock swap of still undetermined value. Boardroom has two computer stores; Gateway has 10.