Guardsmen and Reservist Employer Tax Act of 2003 - Amends the Internal Revenue Code to provide a reserve component employment credit equal to the sum of the employment credit with respect to all qualified employees of the taxpayer and the self-employment credit of a qualified self-employed taxpayer. Limits the credit to $25,000 for each qualified employee. Disallows the credit for failure to comply with reserve member employment or reemployment rights, or when a reserve member is called or ordered to active duty for training.