I used the following tool to extract the icons from GameMaker 8.1, and you will need to download it in order to do this yourself.
http://www.nirsoft.net/utils/iconsext.htmlFollow these steps to create the icon pack.
1) Open LateralGM and set the icon pack to "Custom"
2) Set the folder path for the custom icon pack if you wish, by default I will just leave it to the icons folder next to lateralgm.jar
3) Close LateralGM
4) Create the folder called "icons" from step (2) next to lateralgm.jar, usually in the enigma-dev folder
5) Copy your icons to this folder in the correct structure, e.g, using the same filenames as LGM does. Look at the Calico built-in icon pack as an example, you will need to organize the icons into the folders "actions", "events", and "restree" inside your icons folder.
https://github.com/IsmAvatar/LateralGM/tree/master/org/lateralgm/icons/Calico5) Start LateralGM and the icon pack should have loaded