Here's an excerpt from : Langar - SikhiWiki, free Sikh encyclopedia.
When serving the Langar, the servers must observe strict rules of cleanliness and hygiene. Servers should not touch the serving utensils to the plates of those they serve. When serving foods by hand, such as chapatis or fruit, the servers’ hands should not touch the hand or plate of those they are serving. Those serving should wait until all others have been completely served before they sit down to eat themselves. It is advisable not to leave any leftovers.
Since some Sikhs believe that it is against the basics of Sikhi to eat meat, fish or eggs, hence non-vegetarian foods of this sort is neither served nor brought onto the Gurdwara premises. Others believe that the reason vegetarian food is served in Gurdwaras is so that people of all backgrounds can consume the food without any anxiety about their particular dietary requirement and to promote complete equality among all the peoples of the world. Alcoholic and narcotic substances are stringently against the Sikh diet, hence these with any meat products are strictly
not allowed on Gurdwara
premisesReference:: Sikh Philosophy Network http://www.sikhphilosophy.net/interfaith-dialogues/18095-why-no-meat-served-in-gurdwara.htmlReference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=18095