Hello I am trying to export gif from after effects with media encoder and getting bad quality result

If you are going to create an animated GIF in After Effects there are a couple of design goals that you should follow and a specific workflow that you should follow to export the GIF. 

The design goals should include:

• No duplicate frames - if a part of the timeline doesn't change for a few frames or a few seconds set the duration to 1 frame, you'll adjust the timing of that frame when you prepare for final export
• Make the comp an even number of pixels high and wide
• If you have thin lines against a solid background make sure that the color of the line and the color of the background are a precise color that can be rendered as a gif (265 colors) or the edges may fall apart as the software tries to dither the colors

When you have finished your design add the comp to the Render Cue and use the Lossless or Lossless with alpha presets in the output module. Lossless with Alpha should only be used if you need transparency in the GIF. When the render is finished, open the movie in Photoshop, choose the Video workspace, and set the timing of the frames in your comp. This is where you can tell a single frame to play for 10 or 30 seconds and that is why you do not want any duplicate frames. When you have the GIF set up correctly you can use the File menu to export an animated GIF. This is the only 100% completely effective way to create an Animated Gif in After Effects.