1/ The op says they bought the file, so contained the scope of things to do. Even 45 mb seems wildly too much, given the link they provided. 105 mb just for that seems insane. Think the bigger problem is the overkill probably of both # of triangles & texture sizes. But that’s what happens when you get geometry off of others.
I got some musical instruments from Blendswap. The keyboard was 2.8 million triangles. Fortunately, a lot of it was due to subsurface modifiers, which divided meshes to insane resolutions. Getting rid of them is easy & had an amazing drop to about 160K triangles.
I still had 5 weeks of getting it production ready. In addition to hand removal of un-neccessary geometry, the materials were way too many, and not PBR friendly for export. The keys were also represented as 2 meshes, white & black keys, so it was not playable.
In the end, I got it down to 5,560 triangles, with 4 groups of key instances. Conclusion, arts have very little clue about performance. Getting stuff off them is high quality, and definitely recommended, but plan on spending time editing things down. This squish down, is just a bandaid.
2/ gzip only works on text files I think, but multiple layers of compression tend to no improve things much unless they achieve their results a different way.