That cross is not the right fitting for that location. Also, there is NO need to make your vent lines that big! Air moves much more freely than the liquid waste. Plus, gurgling may actually be an indication of a well-draining line. If it's very quiet, it may not be flowing well.
WRT the slope...as long at any section has at least 1/4" per foot drop, it CAN vary as long as there are no flat spots, bellies, or rises in the pipe run...IOW, it must be a down slope the entire way.