In general, present continuous (or “be going to”) are used to talk about the future when we are certain. Ex: “Tomorrow I’m going to the gym. I’m going to lift some weights. After that I’m working the night shift.” The future simple with “will” is used when we are not 100% certain, with adverbs like “maybe”, “possibly”, “probably” or with expressions like “I think” or “I guess”. Ex: “I think I’ll go to the gym tomorrow. I’ll probably go for a run, and maybe I’ll lift some weights after that.”