An working example of the GStreame 1.18.4 with the new rpicamsrc source.
Please note, it only works on a Raspberry Pi with an 32-bits OS.
If you have an 64-bits OS see https://github.com/Qengineering/GStreamer-1.18.4-RPi_64-bits

To run the application, you have to:

• A Raspberry Pi 4 with a 32-bits operating system.
• GStreamer 1.18.4 installed. Install GStreamer
  
• OpenCV bit installed. Install OpenCV 4.5
  
• Code::Blocks installed. ($ sudo apt-get install codeblocks)
• A working Raspicam

To extract and run the app in Code::Blocks
$ mkdir MyDir
$ cd MyDir
$ wget https://github.com/Qengineering/GStreamer-1.18.4-RPi_32-bits/archive/refs/heads/main.zip
$ unzip -j master.zip
Remove master.zip, LICENSE and README.md as they are no longer needed.
$ rm master.zip
$ rm LICENSE
$ rm README.md

Your MyDir folder must now look like this:
GStreamerTest32.cpb
main.cpp

To run the application load the project file GStreamerTest32.cbp in Code::Blocks.
Next, follow the instructions at Hands-On.

The Raspicam supports many sizes and frame rates, as you can see here.
You can switch between the different options by altering the parameters in the pipeline.
As long it's a valid combination, it will work. For instance:

    //pipeline parameters
    int capture_width = 640 ;
    int capture_height = 480 ;
    int display_width = 640 ;
    int display_height = 480 ;
    int framerate = 90 ;

Since the developer of the rpicamsrc, Jan Schmidt aka thaytan, took the raspivid parameters as best they could, many can now be used in the pipeline. An example;

            "rpicamsrc exposure-mode=off shutter-speed=50 preview=false ! "
            "video/x-raw, width=(int)" + std::to_string(capture_width) + ", height=(int)" + std::to_string(capture_height) + ", framerate=(fraction)" + std::to_string(framerate) +"/1 !"
            " videoscale ! video/x-raw,"
            " width=(int)" + std::to_string(display_width) + ","
            " height=(int)" + std::to_string(display_height) + " ! "
            " videoconvert ! video/x-raw, format=(string)BGR ! appsink";

For more info see rpicamsrc