Here is a full list of all structures in ZScript as of GZDoom 4.2.1. There are 76 structures total. Note that some of these are merely implementation details and should not be used in code.
Struct
├ AutoUseHealthInfo
├ Console
├ ConsoleEvent
├ CVar
├ DamageTypeDefinition
├ DehInfo
├ Destructible
├ DropItem
├ DynArray_F32
├ DynArray_F64
├ DynArray_I16
├ DynArray_I32
├ DynArray_I8
├ DynArray_Obj
├ DynArray_Ptr
├ DynArray_String
├ F3DFloor
├ FCheckPosition
├ FColormap
├ FLineTraceData
├ Font
├ FOptionMenuSettings
├ FRailParams
├ FStateParamInfo
├ FTranslatedLineTarget
├ GameInfoStruct
├ GIFont
├ HealthGroup
├ InputEvent
├ JoystickConfig
├ KeyBindings
├ LevelLocals
├ Line
├ LinkContext
├ LookExParams
├ MugShot
├ MusPlayingInfo
├ OptionValues
├ PatchInfo
├ PlayerClass
├ PlayerEvent
├ PlayerInfo
├ PlayerSkin
├ RenderEvent
├ ReplacedEvent
├ ReplaceEvent
├ SavegameManager
├ SaveGameNode
├ SBarInfo
├ Screen
├ SecPlane
├ SecSpecial
├ Sector
├ SectorPortal
├ Shader
├ Side
├ State
├ StrifeDialogueNode
├ StrifeDialogueReply
├ StringStruct
├ StringTable
├ Team
├ TerrainDef
├ TexMan
├ TraceResults
├ Translation
├ UiEvent
├ UserCmd
├ Vertex
├ VisStyle
├ Wads
├ WBPlayerStruct
├ WBStartStruct
├ WeaponSlots
├ WorldEvent
└ _