Something is wrong with this pull request -- the cell width computation is broken.

..as can be seen in the (now fully-compiling) example from #8:

Ah, right. Quoting http://tex.stackexchange.com/questions/154601/how-can-i-double-dip-environment-contents-to-an-external-file-with-expl3

There is no way to absorb the contents of an environment as a token list other than doing similarly to what the environ package does.

So environ needs to be fixed to stop breaking grid-system.

Unfortunately, this is a dead end.

The token stream save-and-replay functionality that grid-system depends on appears to be extremely fragile -- environ/newenviron conflict with a lot of fairly pedestrian use cases.