To access this window, click 
a node and select Save to Gallery. It can also be accessed for existing gallery entries by double clicking on an entry in the Material Palette tree, or by opening Windows ▸ Gallery Manager and double clicking an entry.
The node gallery provides a way of storing pre-defined configurations of a particular node type, such as the Material Palette which is a gallery of Material SHOPs. This dialog lets you create or edit an existing gallery entry.
Gallery entries can be used to create new nodes by dragging and dropping them from the Material Palette or Gallery Manager into a network editor (or onto geometry in the viewer in the case of the material palette).
Gallery entries can also be used to modify an existing node by opening the gear icon menu in the parm dialog for a node. The menu will list all gallery entries that can be applied to that node.
Parameters
Entry Name
The internal name for the gallery entry. This name must be unique within a particular gallery file.
Label
The label for the gallery entry that shows up in the Material Palette, Gallery Manager, and parm dialog gear icon menu.
Gallery Path
Path where this gallery entry is saved.
Icon
Icon to show in the tree view for the gallery entry.
Allow Icon Regeneration
Refreshes the icon. If the checkbox is turned off, the the old thumbnail is used and is stamped with a badge that indicates the icon has not been regenerated.
Description
The text in this field shows up in the popup tooltip in the Material Palette or Gallery Manager.
Categories
Defines the branches of the Material Palette or Gallery Manager where this entry will appear. It is a comma separated list (which means category names can have spaces).
A category may contain a slash, '/', which is interpreted as sub-category delimiter. The sub-category name to the right of the delimiter will appear as a sub-branch of the main category, which is specified on the left side of the delimiter.
Keywords
A list of searchable keywords that describe this entry. In the Material Palette or Gallery Manager, when a string is entered into the Filter field, that string is matched against the keywords of the gallery entry (as well as the label).
Help URL
URL to show in help browser when F1 or "?" button is hit from the tooltip for the entry in the material palette or gallery manager.
OTL Path
Specifies the path to an OTL file that should be loaded before trying to create a new node from this gallery entry. This is useful if the gallery entry makes use of an asset that is not normally loaded into Houdini but which is used by this gallery entry.
Node Category
Class of node created or modified by this gallery entry.
Node Types
A space or comma separated list of operator type names which specify the type of operator within the operator class that can have this gallery entry applied to them. This is generally going to be a single operator type, but can specify multiple types, including wildcards.
Having multiple types here can be useful, for example, if a gallery entry was created from a very simple material that just contained OpenGL parameters. Then this gallery entry could be applied to any SHOP with OpenGL parameters. All other parameters of this node will remain unchanged.
The Node Type/Node Category pair determine which gallery entries to show in the gear icon menu from the parm dialog.
Set node information/Source Node
Determines what existing node in the current Houdini session will be saved in the gallery entry to define the parameters and contents (if the node is a subnet) that make up the gallery entry. When creating a new gallery entry, this option is always going to be turned on because presumably you want to save some parameter values and node contents to the gallery entry. When editing an existing gallery entry, this option can be turned off so that you can edit the Label or Categories fields without instantiating the gallery entry or worrying about changing the current set of parameters and contents that define the entry.