I made quite some research about this issue, a lot on hydrogenaudio, and right now, for anyone using opus and wanting to get replay gains compatible with other formats (mp3, ogg...) the only solution is to store "old" replaygain tags (replaygain_albumgain and so on...) in the opus files even though that's not recommended.
This message not getting any answer probably indicates that it's not a problem for a lot of people ; but for information and because I stumbled upon it, some guys from the other side of the fence (IETF, the Internet Engineering Task Force) also find this problem annoying (see the email archives
https://mailarchive.ietf.org/arch/msg/codec/mLuGNAYv_GZdaoVcIIL1D8ZQyaE/ if you want to have a look).
I guess this is also a more general problem : very few softwares apparently handle the R128_ALBUM_GAIN and R128_TRACK_GAIN tags. Foobar2000 seems to be the exception.