Can forests with steep gradients be used for forest farming?

Usually, yes. For many plants, slopes are not a problem. It is critical that when cultivating on steep slopes one does not contribute to soil loss or even potentially worse situations, such as landslides. As long as vegetation is currently on the slope, it is possible. Since it is often difficult to access and work on steep slopes, it depends on what plant(s) you are interested in growing and the access to the site.

A practical question will be how to go about cultivating non-timber forest products (NTFPs) and harvesting on a steep slope to avoid soil erosion and damage to the site. To minimize any potential for erosion, you will need to avoid any major soil disturbance while seeding/planting your chosen NTFPs; and you will also have to rely on manual harvesting of the crop when mature. 
In most cases, the criteria for planting come down to soil type, moisture or drainage, aspect, and other plant cover (e.g., trees). These factors could have a greater influence on the decision of which species to plant, and also could help indicate habitat for the plant you want to find or cultivate.
One major consideration is which direction the slope faces. North- or east-facing slopes, most often, are cooler and moister, while west- and south-facing slopes are warmer and drier. For example, some plants, like ginseng, generally do better on north- or northeast-facing slopes.