2G 3G Auto Toggle on Android, is it possible?

Running your android phone on 3G network may drain your battery quickly. Somehow, you think that its possible to change into 2G network automatically.

Then, you look for those applications on playstore, but they only offer manual toggle between 2G and 3G. Thus, battery saver applications can only turning on and off the network connection.

So, what can you do?
Well, you might want to consider GravityBox:
see http://repo.xposed.info/module/com.ceco.gm2.gravitybox

GravityBox is a module which primary goal is to provide the users of MediaTek platform (which is well known for not being opensource friendly)
with a tweak box to turn their vanilla Android stock ROM into custom ROM packed with additional features and tweaks.
Additionally, it should run on any device having vanilla or close to vanilla Android not diverting too much from AOSP (Android Open Source Project).
THIS VERSION IS FOR JELLYBEAN ROMs AND WILL REFUSE TO INSTALL ON KITKAT
For KitKat, use GravityBox [KK] instead.
Main features
- QuickSettings tile management with additional tiles and tile reordering
- CyanogenMod Pie Controls
- Lockscreen targets
- Expanded desktop
- Lockscreen tweaks
- Statusbar tweaks
- Navigation bar tweaks
- Power tweaks
- Display tweaks
- Phone tweaks
- Media tweaks
- Hardware key actions
- Smart Radio
- Ultimate notification control
... and many more
Compatibility
- Supports most of the MTK devices (MT6589/MT6577/MT8389) running ROM based on vanilla Android (AOSP) 4.1/4.2
- Supports other devices running vanilla Android (AOSP) 4.1/4.2/4.3
- DOES NOT support ROMs vastly modified by device manufacturers (HTC Sense, Samsung Touchwiz, MIUI, LeWa, ...)
- SHOULD NOT be used with custom ROMs like CM, AOSPA, RootBox, SlimBean - it makes no sense since the most of the features are already
in the ROM (may cause conflicts and unexpected behaviour)

What you need to have on your android phone are:

1. Your phone must be rooted
2. Install Xposed installer. Link here http://repo.xposed.info/module/de.robv.android.xposed.installer
3. After you install Xposed installer, pick 'Download' and then look for GravityBox. Pick download the newest version and install it.
4. Go back to the front page of GravityBox and pick 'Framework.' Touch Install/Update to activate Framework on your android phone.
5. After that, do not reboot your system first. Go back to the front page and pick 'Modules.' Checklist GravityBox and you may reboot your system.
6. After you reboot the system, you could access GravityBox and do a lot of things such as making transparent  the status bar and notification area. Then you might add more toggle for your settings.
7. Because you want your android phone to be able to change automatically between 2G and 3G, open GravityBox and pick 'Phone Tweaks.' Scroll down until you find 'Smart Radio.'
8. Touch 'Master Switch' to activate the smart radio features. Then choose your normal (pick 3G) and power saving network (pick 2G) mode. Then, cheklist the 'When screen turns off' to activate the power saving mode. Whenever your android phone is on sleep mode, GravityBox will automatically change the network connection from 3G to 2G. Then, when you wake up your android phone, the network connection will be back to 3G.
9. If you thing its too