The West Sussex airport claimed its rival is "likely to fail" in building a third runway even if the project gets the go-ahead.
Concerns have been raised about the environmental impact of increasing capacity at Heathrow, as well as the cost of improving the airport's road and rail links.
In her keynote address to the Tory Party conference in Birmingham, Prime Minister Theresa May confirmed that the government would "shortly announce" a decision on airport expansion.
The Davies Commission recommended in July last year that a third runway should be built at Heathrow.
Other shortlisted options are extending the airport's existing northern runway or building a second runway at Gatwick.
A Gatwick spokesman said the airport would continue to prepare to expand even if it is not initially selected.
He told the Press Association: "Gatwick would also be standing ready to deliver the new runway as should Heathrow get the green light it is likely to fail as it has several times before.
"A decision on next steps will be taken after the government announcement on new runway capacity.
"While Gatwick does not rule out launching legal action, other groups have already said they would if Heathrow expansion is green-lighted."
It is believed that local authorities in areas near Heathrow would consider challenging an expansion decision in court.
The Gatwick spokesman added that the airport will attempt to maximise the use of its existing runway.
"Gatwick would continue to improve efficiency until it reaches 100% capacity," he said.
"One area it would be looking to grow would be its long-haul route network. Having put on 20 new long-haul routes in 2015 alone, Gatwick is expecting to grow the total long-haul routes it offers above the 51-strong network it currently has."
Darren Caplan, chief executive of the Airport Operators Association, the trade association that represents UK airports, has claimed the UK's Brexit vote underlines the importance of a decision being made on expansion.
He told Business Travel News: "Ministers insist that in spite of the UK's impending departure from the EU, the country remains outward-looking and open for business.
"Nothing could better demonstrate they mean that than a decision that would ensure the UK gets the additional capacity it so vitally needs if the country is to maintain and enhance its position as one of the world's leading and best-connected economies."
Caplan added: "It is beyond time for the government to act."
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