Anything you could ever ask for to update your i777 in one place. Download link below...
ROM: cm-12.1-20151213-UNOFFICIAL-i777.zip
Features
Compiled with UBER toolchain (supposedly faster than standard toolchain)
Outgoing call bug fixed (hacked, whatever, it's not there anymore)
TRIM support (thanks to Lanchon)
Otherwise unchanged

CM is awesome.
GAPPS: tk_gapps-modular-pico-5.1.1-20150913-signed.zip
Features
Works.
MODEM: I777UCMD8_modem.zip
Features
Nobody knows, but it's the latest version that I'm aware of.
RECOVERY: SGH-S959G_TWRP_v2.7.1.0.zip
WARNING
This is for the S959G (which I think is identical in every way to the i777). It works on the S959G but I have not tested it on an i777.
XPOSED
So good. Works fine.
You need to install the .zip in recovery, and the .apk normally.
Uninstall .zip included.
Download
Installation instructions can be found on CM's website.
* If upgrading from CM12.1, wipe /data/dalvik-cache first (you probably don't even need to wipe /cache).
* If upgrading from any other ROM, you need to factory reset first (ie. wipe /cache and /data).
Good luck!
XDA:DevDB Information
CyanogenMod 12 [UNOFFICIAL] [UBER] for i777 + Call Bugfix + Goodies, ROM for the AT&T Samsung Galaxy S II SGH-I777
Contributors
dimoochka, Lysergic Acid, Lanchon, razer_kain, rogersb11, cyril279, other amazing devs...
ROM OS Version: 5.1.x Lollipop
Based On: CyanogenMod
Version Information
Status: Nightly
Created 2015-09-26
Last Updated 2015-11-29