The SOGS compression technique works well. You can get 1M Gaussians with full spherical harmonics in about 14MB. There's a good article about it on the PlayCanvas blog:

https://blog.playcanvas.com/playcanvas-adopts-sogs-for-20x-3...

When you say that PlayCanvas is commercial, that's a little misleading. The PlayCanvas Engine (analogous to Three.js and Babylon.js) is free and open source (MIT). The PlayCanvas Engine is where you'll find all the cool 3DGS tech. There are two further frameworks that wrap the Engine (for those that prefer to use a declarative interface): PlayCanvas Web Components and PlayCanvas React. Again, both of these are free and open source (MIT). Only the PlayCanvas Editor (analogous to a browser-based Unity) has optional payment plans (for those that want to create private projects).

PlayCanvas Engine: https://github.com/playcanvas/engine

PlayCanvas Web Components: https://github.com/playcanvas/web-components

PlayCanvas React: https://github.com/playcanvas/react