I admit I don't know what "gmail style" is but it does seem rather limiting in that the relevant letters are split between the Inbox and Outbox for basic one-on-one reply-chains, and it seems if letters are auto-sorted by recipient it further confuses the system. I wonder if the Github link is not the best place to notifiy the team. Maybe if someone filed an issue in the proper place and outlined a reasonable goal and made it request for a company like Opalstack's it could be remedied by the developers.
It seems like the letters that would appear in the thread if they were in the correct folders just need to be "transcluded" (virtually included) in the thread-view and that would solve the basic problem. Plus it would help if selecting multiple letters in the thread could display them in the reading area instead of doing so one at a time. This is just my idea of a full and minimal usability-oriented proposal that could be presented to Roundcube's team.