I've seen many different usage of the word "that" in GMAT. e.g -
1. As a Pronoung
2. As a restrictive clause
& etc

Can some one point me to post explaining the different usage of the word "that" or explain the different usage ?

Thank you in advance.

That girl is beautiful
(demonstrative pronoun)

The house that is red is haunted.
(essential modifier)

hi --

as you just pointed out yourself, the word "that" has a ton of different meanings, so the full breadth of these meetings is not something that would be appropriate for a single forum post.

in general:

if you have extremely general questions to ask about grammar (such as this question, or "what's a modifier?", or other things of similar scope), then you should use a search engine to find sources on the internet.
for any question whose answer is going to run to pages and pages and pages, searching for pages expressly dedicated to that purpose will clearly be better than posting the question on a test-specific forum.

on the other hand, if you have specific grammatical questions that are inspired by, or at least related to, gmat problems, then this is the place to post those.

basically, here's the best way to think about it: if your question is general enough that it would be on a general grammar website, or in a general grammar textbook, then you would be better off looking for complete answers on your own.
if your question is more specific, or more obscure, then that's the purpose of this forum.