Your gain on the sale of rental property is "above the line" and included in the amount on line 6 of the 1040.
Making a charitable donation has no effect on that and does not lower your AGI, as a charitable donation is reported as an itemized deduction on SCH A and is a "below the line" contribution. That means your itemized deductions are included on line 9 of the 1040 (only if they are more than your standard deduction), while your AGI is on line 8. That makes your charitable contributions a "below the line" deduction while your gain on the sale of the rental is "above the line."
