This project is a patched version of GBoard, installable on Android devices.

Tested and confirmed working using ApkTool version 2.5.0

The challenges this project faced:

1. Gboard is a system app, which means it can't be removed from the device, thus, an installation of a patched version is rejected for using the same package name. The package name was changed from com.android.inputmethod.latin to com.android.inputmethod.latinguyk to solve this issue.
2. Gboard performs a signature check on startup, it can be observed in the SignatureUtils.java (Gboard_base/smali/lwe.smali) file. To solve this issue, the new key's hash must be replaced with one of the arrays in the file, as explained later in this file.

In order to install, the package must be self-signed with a new key. Gboard agrees to run only if signed with one of 3 keys, which are hard coded in the code. In order to overcome this issue, one of the hardcoded keys must be replaced with the new, self-signing key used for the patched version.

Get the key hash using the following command:

keytool -list -v -keystore "debug.keystore" -alias androiddebugkey -storepass android -keypass android

It should look something like that:

    SHA256: 1A:E9:B9:06:4B:77:35:F0:5C:0D:B9:A5:1E:1C:6A:78:F3:E5:69:67:07:13:B1:46:E6:29:47:AA:1C:75:52:DE

The following is a small python script to covert the key to the correct representation as used by the smali file.

import binascii

s = b"1A:E9:B9:06:4B:77:35:F0:5C:0D:B9:A5:1E:1C:6A:78:F3:E5:69:67:07:13:B1:46:E6:29:47:AA:1C:75:52:DE".replace(b":", b"")
for i in binascii.unhexlify(s):
    print(hex(i) + "t")

Use the output of this script to replace one of the keys inside the Gboard_base/smali/lwe.smali. Search this file for :array_0 and replace its contents.

If someone wants to write a python script that does this automatically and send me a pull-request, be my guest.

I've used APKLab plugin for VSCode to compile and install the apk on my device. You can also use apktool cli and achieve the same result, just don't forget to sign and zipalign the resulting apk. compile using apktool b Gboard_base --use-aapt2, I've also added --debug but I assume it works without it as-well. The apk will be built into the Gboard_base/dist directory.

You can also take a look at the build.sh script, although I did not use it, it should work. I've included the script for documentation purposes.