In the documentation page for get_magic_quotes_gpc you can read:
Returns 0 if magic_quotes_gpc is off, 1 otherwise. Or always returns FALSE as of PHP 5.4.0.
I guess this can cause the first problem (that your code not works).
You can use code like this:
$_POST = stripslashes_deep( $_POST );
But just check the WordPress docs for the function because you can break functions of the core or other plugins that expect their values slashed.
So it is probably better to work only on your own input vars:
$my_value = stripslashes( $_POST['value'] );