Why gif is not supported ?? it is showing (small) when upload as a thumbnail to my tutotial.

This was reported a few months ago, and it is something GrabCAD is looking into, and considering adding as a feature.
For now it seems that animated gifs do not work in tutorials.

In that case, maybe gifs work in tutorials now.

I had some trouble with gifs in the past if I made them in Photoshop. I had to remake the gifs in Photoscape, and they worked perfect.
Another factor could be the size. Try uploading a small gif as a test and seeing if it works.

GIFs are supported...
Check these tutorials I made:
How to make 3D posts of your models on Facebook
How to write in your mother language, but preview in English
How to prevent models from being cut in PhotoView 360
Even the last one's thumbnail is a GIF...

The issue might be with the size of the GIF; X pixels by Y pixels, not bytes.

It also might be with your internet connection; GIFs take a long time to get uploaded.
Try uploading one GIF at a time, but wait till the loading circle (at the top) disappears, then upload the next GIF.