Two subsidiaries of Geico Corp. posted earnings reports yesterday -- one showing a slight gain in profits but the other disclosing third-quarter and nine-month losses.

Government Employees Financial Corp. lost $1.3 million in the quarter compared with a profit of $474,000 (31 cents a share) in the same period a year ago.

Blaming the red ink on record writeoffs of loan losses, Government Employees Financial showed a nine-month loss of $1.2 million against earnings of $1.9 million ($1.24) last year.

Government Employees Life Insurance Co.'s third-quarter profits increased to $2.1 million (48 cents) from $2 million (46 cents) and its nine-month profits climbed to $6.2 million ($1.39) from $5.9 million ($1.32) a year ago.

Individual life insurance sales and premium income were off in both periods, but investment earnings increased by 18 percent for the quarter and 18 percent for the nine months, the company said.

Atlantic Research Corp. reported its earnings for the quarter fell to $419,000 (21 cents a share) from $484,000 (34 cents) in 1979. Nine-month earnings totaled $1.7 million ($1.01), up from $1.3 million (99 cents).

The Alexandria weapons maker and technology firm's revenues increased to $19.5 million from $14 million for the quarter and to $55.5 million from $39.7 million for the first nine months.

Atlantic Research has changed its accounting method to claim revenues as work on projects is completed rather than waiting until the products are shipped; as a result, earnings are higher than they would have been under the old method. The accounting change added $230,000 to the quarterly profits and $668,000 to the nine-month results. Last year's earnings were restated under the new method, so the comparative figures are higher than previously reported.

Universal Security Instruments Inc., a Maryland manufacturer of security devices, reported fiscal second-quarter earnings of $193,480 (12 cents a share) compared with a loss of $126,516 last year, as sales rose 166 percent to a record $3.4 million for the period ended Sept. 30. Six-month earnings were $317,823 (20 cents) versus a loss of $191,879, as sales rose 124 percent to $6.2 million.