I see...is there just some way to install it with a amd64 deb, a PPA, the classic way?
Yes and no .
First you have to instal WINE. You can find it in the ubuntu repositories or in a ppa like the winehq one.
The ubuntu repositories version is a bit old and the wine hq ones have more up to date versions and diferents flavours (stable, devel and staging)
In the link there is also instruction to do a correct install.
Then you must install foobar.
Download the windows installer and open a terminal in the same folder.
with a simple
wine foobar2000_version.exeyou will be in a standar windows instalation: next, next done.
If you choose standar instalation at the end you will have an icon in the desktop to open foobar2000.
Also are my assumptions that foobar will be applying the RG tags without needing to tweak em like with the Rhythmbox and Banshee RG plugins?
Foobar can scan, apply and play files with replaygain out of the box.
I find it strange that installing this way creates it into a sandboxed wine that is not even installed yet (I'll just install CrossOver for Linux 19, it's better than Wine, bought it dirt cheap at the beginning of the year too.
The snaps are designed for sandboxing.
The wine snap is in fact a foobar installed in wine sandboxed in a snap.
It have some advantages but some limitations too.
If you do like i explain you foobar2000 will work like a normal application.
I'm pretty intrigued with getting RG to play right, and all signs point to foobar to play RG tagged files the way they should be automatically, even if not scanned by the player itself (like I said I did it with metaflac for Rhythmbox and others but it doesn't sound right to me).
Don't worry about that. Foobar2000 will cover all your needs and works pretty well with WINE.
In normal use is like it is a native application.