I didn't want to hijack the first thread on this subject so, how far from house and what if a gas line is going through the gravel near the top? I say near the top, I don't know how deep the utility company buried the flexible line when they replaced the old line a few years ago.
The French drain is laid next to the foundation footing. It needs to be below the basement floor level. But you may not dig below the bottom of the footing of an existing house because that may cause the soil under the footing to slip.
You can dig leaving the gas pipe suspended in mid-air. Just be careful not to hit or bend the gas pipe either when digging or when refilling.
When cleaning anything including baby diapers, it is better to sweep off or scrape off as much solid and semisolid material as you can first. (1) Less cleaning fluid or detergent needs to be used, (2) Less of a mushy mess which in turn means that bleaches or disinfectants will be more effective.