This sentence has nothing to do with measurements...
Grass (a grama) and gram (o grama) are the same word in Portuguese. (Grass is feminine, gram is masculine).
Duolingo had something mixed up. But it's good to learn both words.
Unless it's slowed down, this spoken sentance could easily be "Nossa grama verde" (Our green grass). It's really hard for me to hear the "é" in that sentance. Possibly just a drawback of using a machine voice.
could you translate grama with lawn?
yes. what is all this grass about?
maybe they thought some people would confuse grama with a gram
They are indeed the same word, it's probably disambiguation.
I take it to mean that our field/lot/pasture is green. (green pasture) A lot is a unit of measurement.