Gum File Format

Gum projects are XML, serialized with .NET's XmlSerializer. Prefer editing them with the Gum tool or gumcli; when you must read or hand-edit, this skill covers the structure and the traps. Always run gumcli check after a hand edit — several failure modes below are silent otherwise (see gumcli).

File → root element

Extension	Root element
.gumx (project)	<GumProjectSave> — lists element references, loaded first
.gusx (screen)	<ScreenSave>
.gucx (component)	<ComponentSave>
.gutx (standard element)	<StandardElementSave>
.behx (behavior)	<BehaviorSave>

Element structure

Every file needs the complete root tag with both XML-schema namespaces — the xsi:/xsd: prefixes used on every <Value> are undefined without them. The element has a <Name>, an optional <BaseType> (what it inherits from, e.g. Container), one or more <State>s (the first is Default), and a list of <Instance>s (child objects). A state holds <Variable> entries. This is a complete, valid screen (it passes gumcli check):

<?xml version="1.0" encoding="utf-8"?>
<ScreenSave xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Name>MainMenu</Name>
  <State>
    <Name>Default</Name>
    <Variable Type="ChildrenLayout" Name="MenuStack.ChildrenLayout"><Value xsi:type="xsd:int">1</Value></Variable>
    <Variable Type="string" Name="Title.Parent"><Value xsi:type="xsd:string">MenuStack</Value></Variable>
    <Variable Type="string" Name="Title.Text"><Value xsi:type="xsd:string">Main Menu</Value></Variable>
  </State>
  <Instance><Name>MenuStack</Name><BaseType>Container</BaseType></Instance>
  <Instance><Name>Title</Name><BaseType>Text</BaseType></Instance>
</ScreenSave>

<Name> is the element's name — path-qualified for an element in a subfolder (Elements/Divider), otherwise bare (MainMenu). It must match the reference to it in the .gumx.

Qualified variable names. A variable named MenuStack.ChildrenLayout sets the ChildrenLayout of the instance named MenuStack. No dot means an element-level value. The name before the dot must match an <Instance>'s <Name>.

Parenting instances. Instances are siblings by default — all direct children of the element root. To nest one inside another, give the child a Parent variable whose value is the parent instance's name (Title.Parent = MenuStack above puts Title inside MenuStack, so the container's stacking layout arranges it). Without it, a stacking container has nothing to lay out.

Landmine: enum values are stored as integers

For an enum-typed variable, the Type attribute holds the enum name but the <Value> holds its underlying int (xsi:type="xsd:int"), not the name. WidthUnits = 2 means RelativeToParent. You must know the mappings to read or write these correctly:

WidthUnits / HeightUnits — DimensionUnitType

Int	Name	Int	Name
0	Absolute	6	MaintainFileAspectRatio
1	PercentageOfParent	7	Ratio
2	RelativeToParent	8	AbsoluteMultipliedByFontScale
3	PercentageOfSourceFile	9	ScreenPixel
4	RelativeToChildren	10	RelativeToMaxParentOrChildren
5	PercentageOfOtherDimension

XUnits / YUnits — PositionUnitType

One shared enum; some values apply to the X axis, some to Y. This is not the GeneralUnitType enum you use in C# code — on disk XUnits/YUnits are PositionUnitType. Do not copy int values between the two.

Int	Name	Axis	Int	Name	Axis
0	PixelsFromLeft	X	5	PixelsFromBottom	Y
1	PixelsFromTop	Y	6	PixelsFromCenterX	X
2	PercentageWidth	X	7	PixelsFromCenterY	Y
3	PercentageHeight	Y	8	PixelsFromCenterYInverted	Y
4	PixelsFromRight	X	9	PixelsFromBaseline	Y

XOrigin, and Text HorizontalAlignment — HorizontalAlignment

0 = Left, 1 = Center, 2 = Right.

YOrigin, and Text VerticalAlignment — VerticalAlignment

0 = Top, 1 = Center, 2 = Bottom, 3 = TextBaseline.

ChildrenLayout (on a container) — Type="ChildrenLayout"

0 = Regular, 1 = TopToBottomStack, 2 = LeftToRightStack, 3 = AutoGridHorizontal, 4 = AutoGridVertical.

Landmine: two on-disk variable shapes exist

Gum has written variables two ways over time — a compact attribute form (current tool output) and an expanded element form (older files). Both load:

<!-- attribute form -->
<Variable Type="DimensionUnitType" Name="WidthUnits"><Value xsi:type="xsd:int">0</Value></Variable>
<!-- element form -->
<Variable><Type>DimensionUnitType</Type><Name>WidthUnits</Name><Value xsi:type="xsd:int">0</Value><SetsValue>true</SetsValue></Variable>

Do not hand-craft variables from scratch by guessing the shape. Copy the shape of an existing variable in the same file, or better, let the tool/gumcli write them and edit values only.

Landmine: wrong element names are dropped silently

XmlSerializer ignores XML elements it does not recognize — it does not error. So <States> instead of <State>, or <InstanceSave> instead of <Instance>, loads as an empty element with no warning, and your UI is silently missing content. gumcli check is built to catch exactly this. Run it after every hand edit.

Bottom line

Read these files freely; the tables above let you understand and sanity-check them. But to write them, prefer the Gum tool or gumcli, and validate with gumcli check before trusting the result. Data-model reference: https://docs.flatredball.com/gum/gum-tool/gum-elements.