What is the meaning to close Google apps source code
Technically, you are not allowed to "install Gapps manually" unless your phone already comes with them pre-installed.  The Google Apps package (that includes Google Play Services, Play Store, and several other apps) is Google's intellectual property, and nobody is allowed to distribute it, except for Google's certified partners that have passed CTS.

For example, CyanogenMod ROM does not include, host, or distribute the Gapps package. Instead, they have scripts built into their tools that back up Gapps that are already installed on your handset prior to  installation/upgrade, and restore them afterwards.  Any other ROMs and maintainers who do host Gapps packages are doing so illegally, and are infringing on Google's copyright.  For the most part, Google chooses to not enforce their copyright against small-time guys, but will not hesitate to unleash their army of lawyers when they deem is necessary.

To answer your question, Google's services are essential for their grip on Android's ecosystem. Only the big guys like Amazon or Samsung can afford to completely steer clear of Google's services with their own offerings. For example, not having access to Play Store on an Android handset is a huge drawback for a smaller manufacturer like HTC, Huawei, Motorola, etc., since the alternatives are usually lacking in both numbers, as well as (perceived) security.  Google uses this as both a carrot and a stick.