Gentoo will not put into the tree applications/libraries that are not released and hence there is no libSDL2. What they can do is do live ebuilds (the 9999 variants) but that is a herd call.
Once SDL2 is released then the ebuilds will be updated and the new ABI make variable can be used.
Until then this is an SDL issue not a gentoo or a ubuntu issue. Valve took the decision to release sourcegames prebuild against beta software to get the features they need.