The majortiy of GNOME Circle apps have already been added to nixpkgs but the ones without a tick are missing:

• Audio Sharing audio-sharing: init at 0.2.2 #259976
• Biblioteca
• Cartridges cartridges: init at 2.0.4 #234486
• Chessclock
• Citations citations: init at 0.5.1 #202605
• Clairvoyant
• Collision collision: init at 3.3.1 #195395 collision: init at 3.5.0 #242681
• Commit commit: init at 3.2.0 #172568 commit: init at 4.1 #318705
• Decibels
• Elastic elastic: init at 0.1.3 #226428
• Errands errands: init at 45.1.9 #278951
• Emblem emblem: init at 1.1.0 #213740
• File Shredder raider: init at 1.3.1 #208136
• Font Downloader (tho maybe not, because you're supposed to install fonts the nix way?)
• Forge Sparks forge-sparks: init at 0.2.0 #281571
• Fretboard fretboard: init at 5.3 #281570
• Gaphor
• Health
• Hieroglyphic hieroglyphic: init at 1.0.1 #307857
• Identity
• Letterpress letterpress: init at 2.1 #313689
• Lorem lorem: init at 1.3 #281569
• Metronome metronome: init at 1.3.0 #292974
• Paper Clip paper-clip: init at 4.0 #282070
• Plots plots: init at 0.8.5 #244647
• Share Preview share-preview: init at 0.3.0 #256453 share-preview: init at 0.5.0 #313676
• Switcheroo switcheroo: init at 2.0.1 #282067
• Telegraph telegraph: init at 0.1.8 #282069
• Text Pieces textpieces: init at 3.3.0 #196416
• Webfont Kit Generator
• Workbench workbench: init at 43.3 #208866

(I'm updating this if you inform me of missing GNOME Circle apps in the above list, if PRs are created or merged.)