Here's a list of good apps to install on GrapheneOS. I will also list their sources.
Aves Libre. Gallery app, from F Droid.
Jitsi Meet. Video calling app like Zoom, from F Droid.
Gramophone. Music player, from F Droid.
ConverterNOW. Unit converter, from F Droid.
NewPipe. YouTube client, from F Droid.
Breezy Weather. Weather, from F Droid.
Magic Earth. Maps and OFFLINE navigation, from Aurora Store, OsmAnd & CoMaps, from F Droid.
osm2gmaps. Convert OpenStreetMap links to Apple/Google maps for 'regualar' phone users. Useful to share drop pins, from F Droid.
NotallyX. Notes app, from F Droid.
AntennaPod. Podcasts, from F Droid.
SimplyTranslate. Translation app, from F Droid.
VLC. Video and Media Player. From F Droid.
K-9 Mail. Email client. Works with many providers, from F Droid.
Fossify Recorder. Recorder, from F Droid.
Inter Profile Sharing. Share files across profiles effortlessly, from F Droid.
Image Toolbox. Photo editor, from F Droid.
Open Video Editor. Video editor, from F Droid.
FairScan. Scan documents, from F Droid.
QUIK SMS. SMS texting app, from F Droid. Or Fossify Messages, from F Droid.
HeliBoard. Best keyboard ever, from F Droid.
FUTO Voice Input. Voice typing. No tracking, from F Droid.
All these apps except Magic Earth, are open source. Magic Earth is still a privacy respecting app. NOTICE; Magic Earth recently updated their app, and most features now have to be paid for. Better options now include OsmAnd, CoMaps, and Organic Maps, all from F Droid.