There is no prescribed architecture. The requirements are functional: a room for the Guru Granth Sahib Ji, a place for the sangat to sit on the floor, a langar, and a nishan sahib outside. Everything else is convention.
The convention that developed is a square or rectangular hall with a raised palki for the Guru Granth Sahib Ji, a parikrama for circumambulation, a fluted dome, often gilded, chhatris at the corners, and a deep eave. It borrows from Mughal and Rajput building and it is recognisable anywhere in the world.
The Harmandir at Amritsar set the type: entrances on all four sides, a lower floor than the surrounding ground, and a sarovar. The four doors and the descent are doctrinal statements built into a plan.
The bunga, a tower house built beside the sarovar by a misl or a chief to house their people during pilgrimage, was a Punjabi building type in its own right. Most of the Amritsar bungas were demolished in the 1940s and 1970s to widen the parikrama. The Ramgarhia bungas survive.
Modern gurdwaras are built in reinforced concrete with marble facing and are much larger than they need to be, funded by donation and competing with each other. Very little of the older brick and lime work survives anywhere in Punjab.
The elements are the sarovar, the darbar with the volume installed under a canopy, the langar hall, the nishan sahib, and entrances on more than one side.
The dome, the chhatris and the fluted lotus finial come out of Mughal and Rajput vocabulary, and the style as a recognisable Sikh one was fixed in the eighteenth and nineteenth centuries.
Almost every historic gurdwara has been rebuilt, usually in marble, usually by kar seva, and usually without any record of what was taken down. The architecture is being lost faster than any other category of building in Punjab.