Sangat is the congregation, the assembly of those who sit together. Pangat is the line, the row in which everyone sits on the floor to eat in the langar.
The two words are used as a pair and the pairing is the argument. There is no assembly without the shared meal, and the shared meal is on the floor in a line with no order of precedence.
Guru Amar Das Ji made eating in the pangat a precondition of an audience. Akbar ate in it. That is the standard story and its function is to establish that nobody is exempt.
The sangat also has authority. The doctrine that the Guru is present in the sangat, and the mechanism of the Panj Piare drawn from it, means decisions are taken collectively rather than by an office.
A community with no clergy, no ordination and no hierarchy of religious office is unusual, and the arrangements that replace those things are the sangat and the five.
Sangat is the congregation and pangat is the line people sit in to eat. The two words are used together because the second is what enforces the first.
Sitting on the floor in a line to eat removes the seating order that every other gathering in the region encodes. That was the point when it was instituted and it is still the point.
The rule is that everybody eats the same food in the same line, and it is observed at the Golden Temple by people who would not sit together anywhere else in Punjab.