Yes David, 'take nobody's word for it', and all that.
There is the MGS article, though I did not note which one it was.
And the Wiki article did, I have to say, draw on a more scientific source - the California Department of Food and Agriculture (CDFA). Their page on Gaura provides quite a lot more info -
http://www.cdfa.ca.gov/phpps/ipc/weedinfo/gaura.htm. Actually, therein I was interested to read 'Establishing competitive vegetation may prevent new seedlings from becoming established. However, these species are well adapted to grasslands and can persist in competitive healthy grassland environments.' I can imagine a field of mixed Gaura lindheimeri and grassy seedheads, going very nicely together; prairie community. (I love the prairies when in flower; I remember spending many hours photographing them west of Calgary, though the incessant breeze (swaying the flowers) tested my patience!)
I concede, however, that the CDFA page does not include G. lindheimeri, so extrapolation may bring errors.
Actually, your comments, with special note of dark "Siskiyou", have caused me to put it back into my wants list. I would like to try it on a difficult steep dry slope which needs stabilizing. Rhizomes running amok, supplemented with seedlings sounds like it could prove very effective at holding the soil.