Creating GIF Image from video file is quite simple.

What we will do ?
We will use screencast application like Kazam to record the on-desktop video. Then we'll use mPlayer to capture the jpeg screenshots & then finally we gonna use Imagemagick to convert those jpeg images in GIF.

Tools Required :
- Any Screencast application (I prefer Kazam)
- mPlayer
- ImageMagick
- Linux based distro

Steps :
1) Install required tools[kazam, mplayer, Imagemagick] using apt-get (Debian/Ubuntu), zypper (Suse), pacman (Arch), yum (Fedora) ,etc..

2) Record the screencast & save it in directory (We'll use 'Videos' for simplicity)

3) Open Terminal & type :

mplayer -ao null  -vo jpeg:outdir=mygif

4) Here we have used output directory as 'mygif' so a folder named 'mygif' will be created which will contain jpeg image of each & every frame of the screencast. Now, we will use those jpegs to convert it into gif.
- Open Terminal & type :

convert /* 

Example :

convert mygif/* mygif.gif

- Have patience, it will take couple of minutes in transcoding

5) You can optimise the gif & reduce the file size using 'gifscale'
Open terminal & type :

gifscale -O  -o 

Example :

gifscale -O mygif.gif -o myoptimgif.gif

That's it 👍