After a week and a half of trying every app under the sun to see what works best with the Gemini's landscape format, here's what I've narrowed down as my daily drivers for most tasks:
Launcher: Nova Prime
Calendar: Business Calendar 2 (The free version is fine, but I upgraded to Pro because I liked it so much.)
File manager: Solid Explorer, although ES File Explorer is a close second.
E-Mail: MailDroid (Would be perfect if they'd add an option to swipe to delete -- right now it pops up mail handling options.)
Task Lists: Wunderlist
Web Browsing: Samsung Internet -- Got used to it on my Galaxy Tab and really liked it, and was pleased to see it was available for other devices via the app store. However, I've just installed Microsoft Edge, and it's been very, very good so far -- very clean interface.
Notes: OneNote, Google Keep
Office: MS Office
Remote Access: TeamViewer
Podcasts: Podcast Addict
PDF reader: Xodo, but am not happy with it. Would love something similar to Readdle Documents/PDF Expert for iOS, but haven't found it yet.
Video Player: VLC
Music Player: BlackPlayer EX -- it's alright, but I wouldn't mind finding something better.
Game Emulators: ScummVM, MAME4Droid, most of Robert Broglia's console emulators (NES.emu, etc.), Magic DosBox.
Comic Reader: Perfect Viewer
Ebook Reader: Moon+ Reader
Misc. Must-Have: Ultimate Rotation Control