An employee shall not participate while on Government-owned or leased property or while on duty for the Government, in any gambling activity including the operation of a gambling device, in conducting a lottery or pool, in a game for money or property or in selling or purchasing a numbers slip or ticket. However, this section does not preclude activities

(a) Necessitated by an employee's law enforcement duties, or

(b) Under section 3 of Executive Order 10927 (relating to solicitations conducted by organizations composed of civilian employees or members of the armed forces among their own members for organizational support or for benefit or welfare funds for their own members) and similar agency-approved activities.