Quote:.... I have already tried to change mode but to no avail I'm afraid....
It could be that your gif animation is optimised and another hurdle is, if there is a transparent background.
The sequence then becomes
Open gif. Change mode to RGB
Un-optimize the gif Filters -> Animation -> Unoptimize
Apply the snow effects script.
Then it depends if there is a transparent background or not.
If no transparent background then optimize it Filters -> Animation -> Optimize for gif
Export that as a gif using the disposal method combine.
If there is a transparent background, do not optimize.
Export that as a gif using the disposal method replace.
An example of that https://youtu.be/Nk6JzgvU7x4 duration 3 minutes.