Is the mass gained from steroid use temporary?

Yes and no. Steroids help you in two simple things during muscle growth. First, they get you to your genetic limit faster. It is important to stay active in your training, eat regularly, and follow an effective PCT program to keep you at your genetic limit. Thus, your previous gains will not be temporary.

Secondly, steroids can take you far beyond your genetic limits. However, physical development beyond your limits cannot be maintained long-term without anabolic products. Without Anabolic/Androgenic Steroids (AAS) the body will always return to its normal metabolic limits. In this context, some of the gains will not be permanent.