A word may have many meanings and these meanings develop over the time. Bhai Kahn Singh Nabha has written that the root of the word Langar is Analgreh (ਅਨਲਗ੍ਰਿਹ) which means Paakshaala i.e. kitchen. Langar in Farsi means anchor but I don't think this is the root of the word Langar used in the meanings of Kitchen. In Krishnavtar, there is a Pankiti ਖਾਵਤ ਲੰਗਰ ਦੈਕਰ ਗਾਰੀ he ate Langar while cursing.
It's common to say that "Guru ka Langar" is ready and everyone should leave only after eating it. If we were to interpret Langar as anchor, then what meaning would it have when we say "Guru ka Langar" is ready? If we want to split hair, then we could say that why should say we are eating Parshaada (Roti) when infact there could be no Parshaada in Langar that day and only rice is being served.
Langar means Guru Sahib's kitchen but over time, the meaning of Langar has also become food served at Guru's Langar and for this reason it's not against Gurmat to say that we eat Guru Ka Langar or that Guru Ka Langar will be served Atut - ਗੁਰੂ ਕਾ ਲੰਗਰ ਅਤੁੱਟ ਵਰਤੇਗਾ. It is common to say this - ਗੁਰੂ ਕਾ ਲੰਗਰ ਅਤੁੱਟ ਵਰਤੇਗਾ. Here clearly Langar means food served in Guru's Langar and not anchor or kitchen.
Guru Sahib knows better.
Daas,
Kulbir Singh