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FastStone Image Viewer vs gThumb: catalog facts
FastStone Image Viewer	gThumb
Free Free for personal use. Commercial use requires a paid licence, and the related FastStone…	Free Free and open source under the GPL version 2 or later.
Windows	Linux
Handles every common format including HEIC, WebP, PSD, PDF and camera raw Browser, viewer, basic editor and batch converter in one small program	Native GNOME desktop application Works on local folders without a server
Free for personal use only; commercial use requires a licence Windows only	Linux only No cloud sync or phone backup
Downloadable app	Downloadable app
Closed source	Open source
License not stated	License: GPL-2.0-or-later
No account needed	No account needed
Works offline	Works offline
Official website Full listing and sources Checked September 21, 2026	Official website Full listing and sources Checked September 23, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.