The syntax you are using doesn't work for Oracle. But you can do this using standard SQL:
UPDATE gm_prc
SET RETAIL_DT = '01-JAN-2000'
WHERE EXISTS (SELECT 1
FROM itm_prc_cmp3 itm
WHERE gm_prc.sku_num = itm.SKU_NUM AND
gm_prc.PC_NUM = itm.PC_NUM
);
This should work in both Oracle and SQL Server (which is what your syntax suggests for the original query).