An OxideMod python plugin that writes text to the GUI, c+p from flamingmojo's blog: https://flamingmojo.wordpress.com/2016/07/17/making-a-gui-plugin-the-basics/

announceGui.py

try:
    import Rust, BasePlayer
    import Oxide.Game.Rust.Cui as Cui
    from System import Action
    import UnityEngine.TextAnchor as TextAnchor
except ImportError, e:
    print 'IMPORT ERROR',e
 
 
class AnnounceGUI:
    def __init__(self):
        self.Title = "Announce GUI"
        self.Author = "FlamingMojo"
        self.Version = V(1, 0, 1)
        self.HasConfig = True
 
        self.TINTCOLOURS = {
            'grey_tint'  : "0.1 0.1 0.1 0.7",
            'red_tint'   : "0.5 0.1 0.1 0.7",
            'green_tint' : "0.1 0.4 0.1 0.5",
            'yellow_tint': "0.7 0.7 0.1 0.5",
            'orange_tint': "0.8 0.5 0.1 0.5",
            'blue_tint'  : "0.1 0.1 0.4 0.5",
            'black_tint' : "0.01 0.01 0.01 0.9"}
        self.TEXTCOLOURS = {
            'red_text'      : "0.8 0.1 0.1",
            'yellow_text'   : "0.7 0.7 0.1",
            'green_text'    : "0.1 0.8 0.1",
            'blue_text'     : "0.1 0.1 0.8",
            'white_text'    : "1 1 1",
            'orange_text'   : "0.8 0.5 0.1"}
 
        self.fadeInTime = 0.5
        self.fadeOutTime = 0.5
 
        self.Default = {
            'Version' : '1.0.0',
            'TextSize' : 15,
            'AnnounceTime' : 10,
            'GUIColour' : 'yellow_tint',
            'TextColour' : 'red_text'}
        self.timers = []
 
         
    def OnServerInitialized(self):
        command.AddConsoleCommand("notice.create", self.Plugin, "createNotice")
         
 
    def LoadDefaultConfig(self):
        self.Config.clear()
        self.Config = self.Default
        self.SaveConfig()
         
    def createNotice(self,player,cmd,args):
        if player.IsAdmin():
            msg = ''
            for arg in args:
                msg += arg
                msg += ' '
            tint = self.TINTCOLOURS[selfConfig['GUIColour']]
            txtColor = self.TEXTCOLOURS[selfConfig['TextColour']]
            elements = Cui.CuiElementContainer()
            imgcomp = Cui.CuiImageComponent(Color = tint, FadeIn = self.fadeInTime)
            txtcomp = Cui.CuiTextComponent(Text = msg,
                        Color = txtColor,
                        FadeIn = self.fadeInTime,
                        Align = TextAnchor.MiddleCenter,
                        FontSize = self.Config['TextSize'])
            rectcomp = Cui.CuiRectTransformComponent(AnchorMin = "0.2 0.8", AnchorMax = "0.8 0.95")
            guielem = Cui.CuiElement(Name = "announcementGUI", FadeOut = self.fadeOutTime)
            guielem.Components.Add(imgcomp)
            guielem.Components.Add(rectcomp)
            txtelem = Cui.CuiElement(Name = "announcementText",FadeOut = self.fadeOutTime)
            txtelem.Components.Add(txtcomp)
            txtelem.Components.Add(rectcomp)
            elements.Add(guielem)
            elements.Add(txtelem)
            for player in BasePlayer.activePlayerList:
                Cui.CuiHelper.AddUi(player, elements)
            newtimer = timer.Once(self.Config['AnnounceTime'],Action(self.destroyGlobalUI), self.Plugin)
            self.timers.append(newtimer)
 
    def Unload(self):
        for t in self.timers:
            t.Destroy()
        self.destroyGlobalUI()
                 
    def destroyGlobalUI(self):
        for player in BasePlayer.activePlayerList:
            Cui.CuiHelper.DestroyUi(player, "announcementGUI")
            Cui.CuiHelper.DestroyUi(player, "announcementText")