I'm under the impression that you may be going the wrong way about it. If by gnome-network-manager you are refering to the gnome applet to control Network Manager, why are you using the ifup ifupdown infrastructure at all? They conflict. Comment out the lines pertaining to wlan0 in /etc/network/interfaces and configure your connection with Network Manager applet. Simply select the network you want to connect to in the menu, give the password, store it if you want (it is optional) and let Network Manager do its thing.

I'm under the impression that you may be going the wrong way about it. If by gnome-network-manager you are refering to the gnome applet to control Network Manager, why are you using the ifup infrastructure at all? They conflict. Comment out the lines pertaining to wlan0 in /etc/network/interfaces and configure your connection with Network Manager applet. Simply select the network you want to connect to in the menu, give the password, store it if you want (it is optional) and let Network Manager do its thing.