If you use floor it'll end with "1 seconds.". Actually, there's an issue with it: That doesn't obey the rules of English grammar. Unfortunately, GML doesn't have a C-like ternary operator (`condition ? if_true : if_false`), so you'd have to make it into several lines:

    var t, msg;
    t = floor((global.Server_Respawntime-time)/30);
    msg = 'Respawn in ' + string(t) + 'second';
    if (t > 1)
        msg += 's.';
    else
        msg += '.';
    draw_text(xoffset+10, yoffset+10, msg);