The problem with putting them into HTML and using the Text: HTML filter is that the animation in the GIF is not synchronised with the framerate of your video. so, depending on the GIF animation properties, which will almost certainly vary from GIF to GIF that you are using , you will end up with sub optimal animation.
You should be able to use Shotcutconvert the GIFs individually to “.mov” files. Using these ‘.mov’ files you can anything like lengthen, shorten etc.