That is 100% accurate. You're wrong. Hire a native speaker. What the hell.
Would 家の前に門があります or いえの前にもんがあります be much more common than 門が家の前にあります?
That would be the position of the door emphasized. Which as "there door is in ithe front of the house."
I also agree that "a gate is in front of the house" is contextual the same as "in front of the house there is a gate" が is emphasizing what is in front of the house.
Why does this app not even use the most basic of kanji like 家? Its hard to read.
How does marking 家の前に as the subject make it wrong?!!
Because the subject is the gate in the sentence.
Your phrasing would be ok for 'the front of the house has got a gate' for instance.