I prefer to load the model, add an animation controller and set the correct animation instead of changing the default pose myself. You accomplish nothing by changing the default pose.
https://wiki.jmonkeyengine.org/jme3.html#animations-and-scenes
You might wish to export the weapon as a separate object and attach it to the model.
Learn how to make animations before trying to use them is my last suggestion for now. Heres some links to get you started. Not much is 2.8 specific probably but they are good enough that you can figure things out.
https://www.youtube.com/channel/UCmtyQOKKmrMVaKuRXz02jbQ
This is the first of Sebastians videos.
https://www.youtube.com/channel/UCOKHwx1VCdgnxwbjyb9Iu1g
When you export the models, how you export them makes a huge difference. Each tool has a settings panel, down at the bottom left of the export window.
https://docs.blender.org/manual/en/2.80/addons/io_scene_gltf2.html#gltf-embedded-gltf
Im not certain if you read this yet.
https://wiki.jmonkeyengine.org/jme3/beginner/hello_animation.html
If you read the wiki like a book, i.e. from top to bootm, in order, by the time you hit the end you will be well prepared to write your own game.