1. I don't recommend using gif animations. Gif files are inferior image formats with terrible image quality. I recommend creating your animation frames as png files & then converting them to webp format (webp is a much smaller file size than png, but the quality is almost as good as png when exported at lossy quality 10 (100%). You would take these frames & import them into Visionaire Studio inside of an object or characters animation tab.
2. It's not possible to insert an actual video into a scene object / image. You can only play a fullscreen video. We recommend using mkv (matroska) format for video files. The only way to emulate a video playing inside of a scene object / image would be to use the same method mentioned in (1). Individual animation frames.
Simon_ASA has finished one Myst like game called "ASA - A Space Adventure" using Visionaire Studio already & is currently working on a sequel called "Catyph". Another member called "Paupasia" who joined recently is also working on a Myst like game too. Both of them will probably be able to provide you with more information than I can about making first person adventure games.