Those gr/m2 calculations generally mean a square meter of canopy using SOG techniques with at least an 800 HPS grow light under optimal conditions.

Optimal also includes a highly experienced grower, the best equipment/nutrients/etc and a little bit of luck. Take those numbers with a grain of salt, although I've seen non-professionals outdo those estimates by far when everything goes exactly right.

Generally, breeders use that format for indoor grows and a per plant number for outdoor estimates.
Also, some breeders give those numbers for fresh-harvested yields - others give them for finished product after the cure - some say, some don't.

Your best bet is use those numbers to compare seeds prior to making your choice to see which ones are higher yielders relatively speaking.

Don't be disappointed if your numbers come in lower - and give yourself a major pat on the back if they come in higher.