gn command not found

try to check if the

``` Objective-C
echo $PATH
```

``` Objective-C
/Users/chenyilong/opensource/webrtc_build/webrtc/src/depot_tools:/Users/chenyilong/opensource/webrtc_build/webrtc/src/depot_tools:/Users/chenyilong/.rvm/gems/ruby-2.2.2/bin:/Users/chenyilong/.rvm/gems/ruby-2.2.2@global/bin:/Users/chenyilong/.rvm/rubies/ruby-2.2.2/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/Users/chenyilong/.rvm/bin
```

If you've done this

``` Objective-C
echo "export PATH=$PWD/depot_tools:$PATH" > $HOME/.bash_profile
```

then check the prefix

``` Objective-C
ls /Users/chenyilong/opensource/webrtc_build/webrtc/src/depot_tools
```

if logs like this

``` Objective-C
: No such file or directory
```

this means wrong path.

use the right one instead

``` Objective-C
ls /Users/chenyilong/opensource/webrtc_build/
```

or you can try to install gn command with brew:

``` Objective-C
brew install --HEAD https://raw.githubusercontent.com/Homebrew/homebrew-head-only/master/depot-tools.rb
```

make sure your Termal able to access https://gerrit.googlesource.com

or you will meet this error:

``` Objective-C
Failure while executing: git clone --branch v1.12.37 https://gerrit.googlesource.com/git-repo.git /Users/chenyilong/Library/Caches/Homebrew/repo--git
```

remember those commands:

``` Objective-C
chmod +x src/buildtools/mac/gn
```

``` Objective-C
../../depot_tools/gn gen out/Release-universal --args='target_os="ios" target_cpu="x64" additional_target_cpus=["arm", "arm64", "x86"] is_component_build=false is_debug=false ios_enable_code_signing=false'
```

Start by reproducing the reported `gn command not found` condition with `echo $PATH` and the listed `ls` checks. Review the `depot_tools` path and the Homebrew installation command, then verify that `gn gen out/Release-universal` runs successfully with the provided iOS arguments.

Written by the indexing model from the issue text.