piano chord
B♭6
B♭1D3F5G6
A major triad with a sixth added. The same four notes as a minor seventh chord a third below.
Written the other way round it is A♯6 — the same twelve keys on a piano, a different piece of notation.
What is true of it
B♭6 is B♭, D, F and G — 1, 3, 5 and 6 counted from B♭.
These are also the notes of Gm7/B♭. Which name is right depends on what is underneath and what came before — the notes alone do not decide.
On an instrument that cannot reach all 4, the 5 goes first, in that order.