How to edit gravity?

Your best bet is to insert a BodyForce Instance into one or every of players' limbs.

A local script that demonstrates this.

p = game.Players.LocalPlayer

bf = Instance.new("BodyForce")
bf.force = Vector3.new(0, 400, 0) -- the Y should be edited to your needs. this should also be placed in StarterGui or StarterPack

for i, v in pairs(p.Character:GetChildren()) do
	if v:IsA("BasePart") then
		bf:Clone().Parent = v
	end
end

A server side script that demonstrates this.

bf = Instance.new("BodyForce")
bf.force = Vector3.new(0, 400, 0) --

game.Players.PlayerAdded:connect(
function(p)
	p.CharacterAdded:connect(
	function()
		for i, v in pairs(p.Character:GetChildren()) do
			if v:isA("BasePart") then
				bf:Clone().Parent = v
			end
		end
	end
	)
end
)