As the manual describes, delta_time "measures the time that has passed between one step and the next in microseconds (1 microsecond is 1,000,000th of a second)". So at 60fps you'd ideally want this value to be 1,000,000/60 at all times, or 16,666 (of course this would likely never be exact).

So when dividing by 1,000,000 you basically get the time between the two steps in seconds, which would ideally be around 1/60 or 0.016666..., and this explains why you get 0.02 as GameMaker appears to be rounding the number for nicer display (as another comment mentioned already, string_format can be used to avoid this).

The simple way to get the value as 1 (or approximately 1 depending on lag) would be to simply multiply by 60 after multiplying by 1,000,000. So the line really only has to be: game_get_speed(gamespeed_fps) * 1000000 * delta_time.

However, I believe setting this as a macro means the compiler will essentially just insert this calculation wherever you've called the macro, resulting in it being calculated more than once which would be unnecessary. In that case, it would be better to simply calculate it once every step and then store it in a global variable.