Building G-API with GStreamer support on Ubuntu 20.04:

As a first step, you need to install the following dependencies:

$ sudo apt install libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev gstreamer1.0-plugins-{base,good,bad}

Secondly, you need to launch OpenCV cmake with an extra option specified:
```
$ cmake /path-to-opencv -DOPENCV_GAPI_GSTREAMER=ON
```

After completing these steps, you should see the following in the cmake output:

-- Checking for module 'gstreamer-base-1.0'
--   Found gstreamer-base-1.0, version 1.16.3
-- Checking for module 'gstreamer-app-1.0'
--   Found gstreamer-app-1.0, version 1.16.3
-- Checking for module 'gstreamer-riff-1.0'
--   Found gstreamer-riff-1.0, version 1.16.3
-- Checking for module 'gstreamer-pbutils-1.0'
--   Found gstreamer-pbutils-1.0, version 1.16.3
-- Checking for module 'gstreamer-video-1.0'
--   Found gstreamer-video-1.0, version 1.16.3
-- Checking for module 'gstreamer-audio-1.0'
--   Found gstreamer-audio-1.0, version 1.16.3

And, also the following:

--   Video I/O:
--   ...
--     GStreamer:                   YES (1.16.3)
--   ...

Running G-API GStreamerSource Tests:

When you build G-API with GStreamer support, tests for streaming via GStreamerSource are enabled. However, some of them require test data from opencv_extra repository. So, to launch all the tests successfully, you need:

Clone opencv_extra repository:

$ git clone https://github.com/opencv/opencv_extra.git

Set OPENCV_TEST_DATA_PATH environment variable:

$ export OPENCV_TEST_DATA_PATH=/path-to-opencv_extra/testdata

Run tests with *GStreamer* filter:

$ ./path-to-opencv-bin/opencv_test_gapi --gtest_filter=*GStreamer*

After completing these steps, you should see that all of tests pass. For example:

You may, however, notice some warnings during execution, like these ones:

or

Please don't worry, they are expected!