There are various ways to convert images in gifs or videos with many programs. You can search on google how to do this with other programs that I don't use here.

This is pretty simple:

1. Open the first image in GIMP, then drag&drop all other images you want to add onto the plane to open them as layers. You can use both date/time and numbered renderings. They are already ordered corretly, so they should also be in the correct order after dragging them into gimp.
   
2. Optinal: Rename layers that should show a different time in the gif. For example you want to animate your gif with 10 fps (so 100 ms for each image) but the first and last image should show a second. To do this, change the name of these image layers and add (1000ms) behind it.
   
3. Now export it. Press [CTRL]+[Shift]+[E] or use the file menu. A window will open, choose a filename with the ending .gif and click "export".
   
4. Another window will open. Activate the checkbox "as animation". Now you can set the delay between images (so inverse fps). A useful option to reduce filesize is to choose accumulated frame disposal in the dropdown below and check "Use disposal entered above for all frames".

You can also crop and rescale all images at once when opened in layers like described above.

Note that gifs quality is limited and they reach big file sizes pretty fast. Also loading many images in GIMP takes a lot memory or disk space if you don't have enough. (I crashed my laptop once when trying to open a long sequence of full size renderings and GIMP wanted to eat 19 gb memory).

Ffmpeg is a command line tool and as they say: "A complete, cross-platform solution to record, convert and stream audio and video" (https://www.ffmpeg.org/).
If you have consecutively numbered images (use the file name pattern option to get them easily), you can create a video out of them with a single command. Open the command line interface in the directory of your images or navigate there and use the following command. Replace the framerate and the output name with anything you want. Also replace "image-" (but not the "%06d.jpg") with the first part of the image names. Depending on your system you might also add a complete path to your ffmpeg installation if the command can't be found.

An example for this might be:

Note that all images should have the same resolution. You may need to keep this in mind when using and changing render area limitations ingame.

Other useful commands:
Don't forget to replace the placeholders and settings with what you need. You can find more information about this and a lot more what you can do with ffmpeg on the above linked website.