How can I prevent water from being fed to my foundation through the gas-line ditch?
are you sure they haven't damaged a drain or water inlet line in the process of installing the gas?
if a water inlet line was damaged and is on the inside of the meter it would spin the meter slowly (over say an hour or two) when you don't use any water in the house for that time.
i would call the people that did the gas pipe install and ask them to find and fix. they're covered by insurance..and accidents happen.

[edit] maybe they hit a drain pipe instead.
whatever the problem, they should fix it. you can't install a pipe in a way as to create a ditch that would cause moisture problems under normal conditions, which includes normal rain. to cause problems the ditch would need a considerable amount of bedding screenings and wrong slope, to start causing problems straight away just from rain infiltration into soil. or do you get five inches of rain each time?

you can't interfere (change) the bedding material (e.g. pouring in cement) so alternatives like french drain or deep auger holes (only if hydrology of the soil allows) are hard work and expensive.