How do you insert the players username in a custom npc dialogue gui?

Since you want to get the player's username in the text, you want to use concatenation.

Concatenation is grouping or combining two strings together.

--Get Player
local Player = game.Players.LocalPlayer

--Reference Text Label
local TextLabel = script.Parent.TextLabel

--To Concatenate the Player's Name to the Text Label you would want to do this:
--REMEMBER: THE PLAYER'S NAME IS A STRING, BUT NOT THE PLAYER!
TextLabel.Text = "Hello there "..Player.Name..", how are you doing?"

Here is another example on concatenation.

--Get Player
local Player = game.Players.LocalPlayer

--Reference Text Label
local TextLabel = script.Parent.TextLabel

--Lets Split It Up To Multiple Strings to make it easier to understand)
--Rather than do what I did in the top, lets split the entire sentence into multiple strings.
local string1 = "Hello there"
local string2 = ", how are you doing"
--To Concatenate the Strings together, in Lua, you use ".."
TextLabel.Text = string1..Player.Name..string2
--This will result in the same output as the example previous example.

arson has a good answer. if you want to get more complicated eventually, it can be easier on you to use string.format(). here's an example of what you would want to do using formatting instead.

local player = game:GetService("Players").LocalPlayer
local greeting = "whaddup %s, how's it going?"

print(string.format(greeting, player.Name))

-- these also do the same thing
print(greeting:format(player.Name))
print(("whaddup %s, how's it going?"):format(player.Name))

%s gets replaced with the player's name.

if you read the link I gave, you will find that you can do this with multiple strings, numbers and more, so it is probably good if you are writing a lot of dialogue. that said, concatenation is also fine for simpler phrases. or like, if you don't care.