What is the grounding requirement one a second sub panel in a steel building
I'm assuming this new panel is in the same outbuilding shop as the 125A panel.

Any sub panels fed from the "main" panel for a structure (in this case the existing 125A panel is the main panel for your shop) should be grounded with a dedicated grounding conductor in addition to the hot and neutral conductors. So yes, you must run a ground wire to the new panel.

Note that connecting your panels to other metallic things like the metal building (or copper pipes, etc.) does NOT count as grounding. From a safety standpoint they are irrelevant. Your metal building is probably grounded to your existing panel/ground, not the other way around.