The average price of gold fell to $199.05 an ounce yesterday's Treasury auction, the first since the administration announced its tough anti-inflation policy. That represents a drop of nearly 13 percent from the previous sale in October.

The largest single purchaser was Swiss Bank Corp. of Zurich, which bought 243.000 ounces. Republic National Bank of New York was second with 120.000 ounces. In all, there 24 parties bid for 911,600 ounces; 750,000 ounces were offered for sale yesterday.

Proceeds totaled $149.3 million, of which $31.7 will be used to retire gold certificates.

Next month, the Treasury will begin an unspecified number of monthly auctions, offering at least 1.5 million ounces at each sale. The exact amount will be announced a month in advance.

Until now, the gold offered for sale has been of 99.95 purity. Starting Jan. 16, some 500,000 ounces will be offered in bars with a fine gold content of 90 percent, while the remainer will be 99.5 percent pure. The bars, which were formed by melting down coins many years ago, contain an alloy. However, customers will pay for only the actual amount of gold.

In each of six previous auctions this year, 300,000 ounces of gold has been sold. The fivefold increase is an indication of the administration's policy of driving the price of gold down by increasing the supply and consequently increasing the price of the dollar.