I just moved my mod to 1.8.9 and recommended Forge 11.15.1.1722.

In my gradle build file the mapping evaluates to

minecraft {
   version = "1.8.9-11.15.1.1722"
   mappings = "stable_20"
}

but I get a warning

This set of MCP mappings was designed for MC 1.8.8. Use at your own peril.

in the gradle log

 

Should I use a "snapshot_xxxxxxxx" mapping?

Latest Mincraftforge build seems to use "stable_20" as well.

For some reason though, compiling a mod with the stable_20 mapping does not work.

I tried using the  following version:

minecraft {
    version = "1.8.9-11.15.1.1722"
    runDir = "run"
    mappings = "stable_20"
}

I simply copied this from the forge mdk  I downloaded. When I tried compiling my Mod, I got this error:

Building Mod File...
To honour the JVM settings for this build a new JVM will be forked. Please consider using the daemon: http://gradle.org/docs/1.11/userguide/gradle_daemon.html.

FAILURE: Build failed with an exception.

* Where:
Build file 'C:\Users\~~~~~\Desktop\Modding\Applied Science\Applied Science\build.gradle' line: 34

* What went wrong:
A problem occurred evaluating root project 'Applied Science'.
> This mapping 'stable_20' exists only for MC 1.8.8!

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 10.553 secs

Is there another mapping I can use instead?

 

Changed to stable_22, tried again, got an exception. Reinstalled Forge using stable_22 as well, tried again, got an exception.

Obviously, some important files were not added during Forge installation?!

 

Building Mod File...
To honour the JVM settings for this build a new JVM will be forked. Please consider using the daemon: http://gradle.org/docs/1.11/userguide/gradle_daemon.html.
****************************
Powered By MCP:
http://modcoderpack.com/
Searge, ProfMobius, Fesh0r,
R4wk, ZeuX, IngisKahn, bspkrs
MCP Data version : stable_22
****************************
:compileApiJava UP-TO-DATE
:processApiResources UP-TO-DATE
:apiClasses UP-TO-DATE
:sourceMainJava UP-TO-DATE
:compileJava UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:jar UP-TO-DATE
:compileTestJava UP-TO-DATE
:processTestResources UP-TO-DATE
:testClasses UP-TO-DATE
:test UP-TO-DATE
:extractMcpData UP-TO-DATE
:getVersionJson
:extractUserDev UP-TO-DATE
:genSrgs FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Some problems were found with the configuration of task ':genSrgs'.
> File 'C:\Users\~~~~~\.gradle\caches\minecraft\net\minecraftforge\forge\1.8.9-11.15.1.1722\unpacked\conf\packaged.srg' specified for property 'inSrg' does not exist.
> File 'C:\Users\~~~~~\.gradle\caches\minecraft\net\minecraftforge\forge\1.8.9-11.15.1.1722\unpacked\conf\packaged.exc' specified for property 'inExc' does not exist.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 13.782 secs

OK, I now deleted the .gradle folder from my user account, reinstalled forge, installed gradle 2.12 as the compile sequence requested at least 2.3, compiled and got no error. I wonder why it crashed before.

I also deleted the .gradle folder from my user account, reinstalled forge and installed gradlew 2.7 (that's what the output on the command line was) and I got the same error again.