Is it not correct "אני חושב על גילה"?
Of course it is. We tried advocating the use of common, spoken Hebrew, but it should still be accepted. Report if that's not the case.
Is it possible to report after the exercise has been already done?
Not that I know of, but don't worry about it, it will come up eventually.
"אני חושב על גילה"
זה גם תשובה נכונה!