Langar doesn't have to be in a Gurdwara. The purpose of the meal is to serve food to all (especially the needy), and for even the privileged ones in the society to sit down and have the same food in the same conditions (even the richest person should be in the same queue and have Langar with people from all strata of society) to promote a sense of equality. One of the major Langar charities, The Midland Langar Seva Society (https://www.midlandlangarseva.org/), does it on the streets, as do many others. The first ever Langar by Guru Nanak (the founder of the religion) was on the street as well. But, it's kind of hard to tell (without more details) whether what you had was langar. And, it doesn't really matter whether it is Langar, because it serves the same purpose, and the people doing that are definitely doing a service to the society the same way.

When it comes to using charity as a tool to lure people into some ideology is what puts me off. Ideally, charity should be done to help others, and not for self-promotion or for the promotion of an ideology is what I've learnt as a Sikh, and would like to retain the same belief even if I'm not a Sikh some day.