The reason to construct an 'apron' for the gabion, is to prevent undercutting of soil, immediately on the downstream side of the gabion. Without an apron, water flowing over the top of the gabion would eventually cause the gabion to collapse into the eroded hole and over time, erosion would continue and the gabion, in effect, would appear to roll down the wash and disintegrate.
To construct the apron, wire mesh was first laid-down across the wash and up each side of the wash by a few feet. The wire mesh was then wired to the gabion.
Approximately 12-inches of river rock was applied to the base of the apron. The rationale behind the application of river rock, is to slow the velocity of any water that migrates downward as water flows over the top of the gabion. Note: This approach is experimental: I had river rock left over from other another project.