This excerpt taken from the GPS DEF 14A filed Apr 16, 2008.
Eligible employees voluntarily elect whether or not to enroll in the ESPP. Employees join for a purchase period of three months. Employees who have enrolled in the ESPP in previous periods will continue to participate in future rolling three-month periods. However, an employee may cancel his or her participation at any time (subject to ESPP rules). Employees contribute to the ESPP through payroll deductions. Participating employees generally may contribute up to 15% of their eligible compensation through after-tax payroll deductions. From time to time, the Company may establish a lower maximum permitted contribution percentage or change the length of the purchase periods (but in no event may any purchase period exceed 27 months). After the purchase period begins, employees may not change their current contribution percentage until the next enrollment period.