New to digital art. Uploaded my drawing into digital. Trying to put color gradients on it. Can't figure out how?

So I have been wanting to make T-shirt designs and stuff like that. I drew one by hand and it is basically a sketch. I have uploaded it into multiple different digital art programs including Canva, Ibis paint, Sketchbook and others. All of which have said they can do color gradients. Some of them I couldn't get to do anything. Some of them could only do their own gradients and were only the full screen whereas I only wanted the parameters within my design to have the gradient. Please help me figure this out. What am I missing here?