HDRIs:

Improved compatibility with Poly Haven add-on: You can now select your Poly Haven asset library as a source folder for Gaffer, and only the HDRIs will be detected.
New "Favorites" feature - mark your favorite HDRIs, and easily filter to them later.
New Folder filtering - show only HDRIs in specific folders/subfolders to make it easier to find what you're looking for.
Added Color tinting option, allowing for the environment to be tinted any color, with any blending mode, and also for the background to be controlled separately (e.g. to make the background a solid color while still using an HDRI for lighting).
You can now adjust the rotation of the background separately without affecting lighting (and optionally reflections).
HDRI settings are now stored per-world, allowing you to use multiple World settings as a kind of HDRI template/snapshot system.
Support for HDRI libraries that may not always be available (e.g. on external/network drives).
The current HDRI is no longer changed automatically every time you search for something, it stays the same as long as the current HDRI is in the search results.
Added offline mode to disable fetching Poly Haven HDRI tags.
Advanced settings are now in a standard sub-panel instead of a box layout, for better consistency with the rest of Blender 4.1's UI.

General:

The light list now auto-refreshes whenever lights are added, removed, or renamed, so you won't need to manually refresh it as often. This may affect performance in scenes with many thousands of objects, so it can be disabled in the preferences.
New -/+ buttons next to light strength sliders - to conveniently half/double the light power. Hold Shift for smaller increments, and Ctrl for larger increments.
For multi-user lights, the number of users is now indicated in a little button next to the name. Clicking the button copies the common object name to the data name (i.e. light data or material name) to the object names to help you keep track of which objects are using the same light data. Shift-clicking it does the opposite, renaming objects to match the data name.
Renaming of lights from the light list is now streamlined (no popup required), as long as auto-refreshing is enabled.
Add settings for Fast GI in Cycles since Ambient Occlusion was removed.
Show light shape selector for Area lights.
Remember state of world ray visibility (Camera/Diffuse/Etc.) when toggling world visibility or Solo-ing any light.
Catch some JSON decode errors in case of corrupt data files.
Many functions are now undo-able
General compatibility fixes for Blender 4.1