How to use gsub in InputBox?

To answer this question, I think it's necessary to explain how string patterns [1] work in Lua. In this case, your code takes a string pattern ("%D" .. "%P", which is effectively the same as "%D%P") and applies it with string.gsub to remove anything which doesn't match it. Let's break it into pieces:

%D: Matches anything which isn't a digit.
%P: Matches anything which isn't a punctuation character. Punctuation characters are !@#;,. [2].

When these two patterns are concatenated into %D%P, the pattern matches any non-digit followed by any non-punctuation character. What you want instead is to match all non-digits and periods as a single character, not followed by each other. The correct way to do this is to define a character class set [3], which contains every character except for digits and period. The string pattern for it is the following: [^0-9.]

Basically, the square brackets [] delimit the character class set, 0-9 means it should match digits, . means it should match a period, and finally ^ means that it should be negated, i.e. it should match everything except for the aforementioned characters.

So, your code would look like this:

local inputbox = script.Parent.Parent.TextBox

inputbox.Changed:Connect(function()
    inputbox.Text = inputbox.Text:gsub("[^0-9.]", "")
end)

Disclaimer: untested.

[1]: Programming in Lua: String Patterns
[2]: Roblox DevHub: Character Classes
[3]: Roblox DevHub: Character Class Sets