(The wealth of langgar was distributed including kheer which had generous portions of ghee).
Article 21 of the Sikh Rehat Maryada makes some references to langgar but that provision does not answer your query.
My personal view is that if langgar is prepared by Sikhs at home or elsewhere after Ardas and the substance/essence of the pangtis quoted above is observed, there should be no objection to it being served as Guru Ka Langgar. Only the place of preparation is different. If all other protocols are observed, re cleanliness, vegetarian food, and Ardas before preparing and Ardas in shukrana after preparation and seeking the Guru’s agyaa to use it as Guru Ka Langgar, my view is it can be used as Guru Ka Langgar.
This is in contrast to food bought from any shop. There is no guarantee under which conditions such food was prepared.
Despite my views, Sikhs generally are reluctant to treat food brought from outside as Guru Ka Langgar.
Sometimes it may be practical to prepare the langgar in one gurdwara kitchen and to transport the langgar to the gurdwara where the prayers are to be held. There can be no objection to langgar brought in from another gurdwara kitchen.