How can you avoid pixelation in creating GIF using photoshop?

There is less chance to avoid that in photoshop, You may can use Adobe Fireworks or Google Web designer. 

Over the last couple of years, there has been an increasing amount of overlap in the functionality between Fireworks and both existing and new programs like Photoshop, Illustrator, and Edge Reflow. At the same time we have shifted to focus our engineering teams on building smaller, more modular, tools and services for specific tasks in web design. Due to this overlap and as well as our change in our product development focus, we have decided not to update Fireworks to CC and instead will focus on developing new tools to meet our customers needs.

One of the methods to do so is to increase the number of colors, 256 would offer less pixelation than 8 or 16-bit when saving image for web, another option is changing the dither options, it would increase the image in file size but reduce its pixelation.

Over the last couple of years, there has been an increasing amount of overlap in the functionality between Fireworks and both existing and new programs like Photoshop, Illustrator, and Edge Reflow. At the same time we have shifted change in our product development focus, we have decided not toupdate Fireworks to CC and instead will focus on developing new 

Select Image on the menu bar/ Image size. Given the proper pixels of width and height (each platform demands different sizes for posting, use it on the profile photo or for de heading), we Save for Web to obtain a low resolution .jpeg image