I guess the 'truck-stair' variant in GSX is not correctly modeled to meet the front stair of a 737 type.
Please stop repeating the GSX stair is "not correctly modeled", since it clearly is but, of course, it must work within the constraints of the MSFS SDK.
As you can see from your own pictures, to meet the door of a 737, the stairs have a slightly pitch down attitude, enough to cover the 16 cm we need to reach the 737 front door.
The issue is, the MSFS SDK requires the staircase animation to have two parts: the first one is used only to tilt up the stairs to have their top platform level with the ground, the second one is the one that reaches the door. And as I've said already, the
correctly modeled stair, is already at 2.95 mt. from ground at the minimum height at the start of the 2nd animation, so it's 16 cm too high, which means GSX will discard it as not being suitable for the front door of the 737.
If we cheated, and lowered the height restriction, the stair will appear but, the walking top platform will be 16cm higher than the door threshold, which wouldn't look very good with passengers walking.
Or, we could cheat in another way, and resize the correctly modeled stairs to be like 95% scale? I guess nobody will notice and everybody will be happy, right?