Take this with a pinch of salt as I haven't done the Gender component and it also depends on how many marks the question is.
"Describe a study which has investigated gender schema theory" would likely just require you to do what it says on the tin, pick a study that investigates gender schema theory and write a paragraph describing the aims, methods, results, and conclusions of the study (I'm assuming this is like a 4 marker). If it said "Describe and evaluate" which would probably be a 16 marker, do that, then add 3 paragraphs of evaluation.
"Outline gender schema theory as an explanation of gender development" would be similar, but with different content - again, if it just said "outline", just do a paragraph outlining the theory itself, just what it is. If it says "outline and evaluate", then write that paragraph, then three paragraphs of evaluation.
Basically the difference is, in the first question you're describing a study which has investigated gender schema theory, and in the second question you're describing the actual gender schema theory itself.
Again, I don't do gender in my psychology course, but this is what I'd do if faced with this problem in any other component I did. Hope this helps!