Visit the PHP documentation:
PHP: $ _GET - Manual
As for it already being catching the ID by GET, if I understood correctly, you can use another variable to pass other values, separating them by the character & . GET retrieves values through the page URL:
http://www.pagina.com.br/index.php?id=1&outra_variavel=valor
When loading this page, to capture the value of the parameters "id" and "variable_variable", use:
$idProduto = $_GET['id'];
$outroValor = $_GET['outra_variavel'];
NOTE: If you are dealing with sensitive data such as a password, use the POST method and, if necessary, encryption (md5).
I do not know if it helps, what your level of knowledge, or where it's in the project, so that's what I can do right now. Good Luck!