Since you already have a thread running about this subject, it would be best to declare one of the threads dead and use only the remaining thread.

I accidentally posted it twice...I have requested that the other one be deleted. Thanks.

Since you just finished the retainer wall, you might not like my advise...but I think it is too narrow. It's splits an already narrow space in 1/2.

My vote would be to bring that retainer wall out almost to the fence, then add a narrow evergreen hedge. could be yew, boxwood, or arborvitae. Just pick a narrow cultivar. for boxwood consider Buxus sempervirens 'Dee Runk' and for arborvitae Thuja occidentalis 'Degroot's Spire'. Yew would probably be my last choice, but something like Taxus x media 'Hicksii' would work ok. Yew can be sheered like crazy, so you can get something that might grow wider and faster and keep it under control if you wanted. Yew is one of the few conifers that will actually regenerate growth off of old wood.

In front of the wall you might be able to plant a flowering shrub or small tree... not sure what is there based on the picture though.

Finally, I would rule out grass... you would never be able to get it grow in the gully.