I'm a bit late, but if you are using Windows, here's another way:
Load your gif via File > Load.
Click to save in File > Save as.
Under Gif options, enable transparency.
Select the color to be treated as transparent via a color picker.
(PS: I'm the developer)
This is a really interesting question. And will be a followup from this one
How to make a given color a bit darker or lighter?
I will not address how to mask properly an aliased border, I will talk about colors and opacity components.
Let's explore how theoretically the colors will respond as transparent over a white or black background.
I stumbled upon this superuser post that seemed related and was able to make the GIMP instructions work reasonably well
Open the image in GIMP 2.10
Right click the image and choose Layer >> Transparency >> Color-to-alpha
The default white choice was nearly exactly the effect required, though the red appeared slightly transparent. I was able to ...
One of the possible solutions for this is maybe using a free background remover tool. It can automatically remove background image quite well, it can produce slight transparency blur around the edge exactly as you mentioned.
(Inspired by previous answers from xenoid and Reece.)
Adjust the input levels of the alpha channel of your selection. To do this, start with your selection, then:
Colors menu > Levels... > Adjust Color Levels window:
Under the Input Levels section:
Optional: click the black eye-dropper, then click an area of your selection that ...
Gradient and opacities using layer masks
Select the layer with the lenses. Create a selection of the lenses. I did a quick select with the magic wand tool.
With the lens layer selected, press CTRL + J (Windows) or CMD + J (Mac) to create a copy of the selection onto a new layer.
Select the new lens layer, CTRL + Left Click (Windows) or CMD + Left Click (...
The short answer to your question would be to include a step between making your selection and inverting it: choose Select > Modify > Feather, and choose a 1~2px value.
Do yourself a huge favour, however, and rather than inverting your selection and hitting Delete, click on the 'Add Layer Mask' icon in the Layers palette. It looks like a rectangle with ...