How to check every value with GetChildren?

You have the right cocept, just you're using the return statement inproperly.

The return statement is a statment that should be used in a function. It makes whatever function you used the statement on return the given argument, and also discontinues the function. Example;

function add(a,b)
	return a + b
	print('lol') --This won't print
end

print(add(1,3)) --> 4

But that's besides the point. Instead of returning false, set the value to false.

local gather = workspace.Model1:GetChildren()

for i = 1,#gather do
	if gather[i].Value == true then
		gather[i].Value = false
	end
end

Additional changes - you can use a generic for with the Pairs function.. which is actually meant to iterate through tables.. also you can 'toggle' the value by doing value = not value.

local gather = workspace.Model1:GetChildren()

for i,v in pairs(gather) do
	v.Value = not v.Value
end

--And if you don't want to toggle it then just return it to the old syntax(:

for i,v in pairs(gather) do
	if v.Value then
		v.Value = false
	end
end