What does m% stand for?

It must be a per cent mille (milli per cent) or 10⁻⁵. I have no idea why TI decided to use such an obscure unit in a data sheet. It is occasionally used in other fields of engineering, but I have never seen it used in electronics.

Assuming you're talking about figure 33, looks like they're plotting 1000x the error in %. So milli-percent.

Maximum gain error is 0.047% so that would be 47, which fits with the graph.