$query="SELECT gmtbl WHERE UID='$_SESSION['id']'";
If UID is a numeric, integer field, you should not have quotes around the value
UID = $_SESSION['id']
Also when using indexed array inside "SELECT double quoted string"
you should add {curly braces} around
So, my guess is this would be right for your query. (UID is INTEGER)
$query="SELECT gmtbl WHERE UID = {$_SESSION['id']} ";
But if your UID is a VARCHAR or TEXT field:
$query="SELECT gmtbl WHERE UID = '{$_SESSION['id']}' ";