The Overview describes this as total steam injected into a sand. We are already given this with the steam injected with the three injectors for a well. Its value is of the same order of magnitiude, but somewhat larger. I verbalize this in my head as “total gentle injection” which is, of course, meaningless.
What does ‘GNTL’ signify? One conjecture of mine is Gas NaTuraL, natural gas or methane, a natural feature of most (all?) oil fields. This could be recycled endlessly, separated from the pumped oil and then reinjected.
Does anyone know? Any ideas? I feel dumb for not having thought of this when speaking with the Aera people,

I believe there may be more than 3 injectors connected to the same sand. This is why sum of SGMT_CUM_STM_INJ_i does not always add up to TOTAL_GNTL_INJ, but is often smaller.

That is correct! The three injectors shown are representative. There may be additional injectors.