First, I agree with everyone - safety first - contact the utility company and have the underground utilities marked. It is no fun when all your neighbors have to be evacuated while the fire department and gas company show up to make repairs. People tend to look at you funny after that happens. (Two of my neighbors made this mistake.)
My suggestion is slightly different. I would not dig into the ground at all. I would build a garden box...
First I would add a double layer of cardboard or landscape fabric on the ground. Here is an up-side-down image of the one I am working on for a slope in my yard.
The 'top' is the wooded board and the 'bottom' is some plastic re-purposed landscape edging. (You may want to use all wood or all plastic as it would have a better appearance; I just used what was on hand.)
Once the box is complete, set it on the ground, and use a level to determine how much of the plastic (or bottom board) needs to be cut away to make the box fit the slope.
My box is about 10 inches deep at the shallow end, but yours can be any depth you choose. The sun/shade, depth of box, etc. will determine which plants can grow in your spot.
My suggestion also allows for (somewhat) easy removal in the event that repairs to the gas line need to be done is the future. Depending on the sun/shade I think that herbs and flowers with shallower roots would be a good choice.
It was necessary for me to develop this strategy when I learned that anything planted 'in the ground' belongs to the landlord. All of my newer plantings are in these boxes and can be moved if needed.
The 'top' is the wooded board and the 'bottom' is some plastic re-purposed landscape edging. (You may want to use all wood or all plastic as it would have a better appearance; I just used what was on hand.) 
