Medium gravel. Drainage is your friend here.

Ditto. And maybe a drain tile to get the water out of the area. Without drainage, freeze/thaw will push the wall out over time.

I recommend using a hole saw and then inserting 2" PVC pipe through the wall about every 4 feet, with drainage cloth over the end of the pipe that is inside. Then backfill that area with gravel, also wrapped in drainage cloth. This will allow water to seep down and exit through the drain pipe, instead of building up behind the wall and pushing it out.

They should have also put some type of "deadman anchor" for the retaining wall to prevent the dirt from pushing it out. This is a picture of one example. https://www.pinterest.com/pin/431853051751958426/

Bruce

Who builds a retaining wall and doesn't fill behind it? I hope this wasn't a professional landscape/construction company.