Hello dear anon :)
Let’s see. I personally use CC, and that is where I make my gif’s, but I don’t think it is a big difference, but then again, I am not sure. Have the quality always been bad? And is it bad already before you save them, or is it when you upload them to Tumblr? Well, anyways, this is the things that CAN be the issue, so try that first, if not, please send me an example of the gif so I can see what kind of bad you are talking about :)
1. The video you are making the gif off is of bad quality. If you have taken the video from YouTube, it should be downloaded as a HQ file, 720p or 1080p. If it is smaller, the quality is for the most time very bad, and cannot be used unless you are very good with Photoshop. (You may be able to use 480p if you are making photo-sets with small pictures, though)
2. If the gif isn’t that bad until you put it in Tumblr, it may be the size of the gif that is the problem. Click alt+ctrl+I (or command+alt+I on mac) and look at the width. If you are planning on filling your whole tumblr post, the width should be 500px for the best quality. If it is smaller, tumblr will automatically resize it, and the quality will drop drastically.
3. The problem can be when you are saving the gif. On the picture below (ignore the fact that my photoshop is in Norwegian…), you see the “Save for Web”-box. In the red box marked with 1, you will see how many colors are saving with the gif, this should be on max: 256. If it is not, then the colors of the gif may seem way off. If you see in the second box, in english it’s called Dither, and I am not a hundred percent sure what it does, but it should be on 100% for maximum quality. In the last, you can again see the size of the gif, and as said before, if you are filling the whole Tumblr-post (like mine) then the W should be 500, but you cannot change this in the end, that will make the gif bad again..
I hope something of this helps, and if not, please feel free to ask again, and then please add a link to one of the gifs that doesn’t look like it should :)