Where did Staoshi Nakamoto stated that bitcoin is gold or digital gold?
Not in any of his public writings.
an argument that Bitcoin is a store of value and not a meant of payment.
In his original whitepaper, Satoshi Nakamoto was very clear that they saw Bitcoin as a means of payment.
What is needed is an electronic payment system
The whitepaper does not mention the phrase "store of value". Since they were undoubtedly aware of this property of currencies, we must infer it was not of primary importance in the motivation for the design of Bitcoin.
So it would be surprising if there is any evidence available that would conclusively prove that Nakomoto completely changed their mind about this.
See also Breakdown of all Satoshi's Writings Proves Bitcoin not Built Primarily as Store of Value
Did Satoshi build Bitcoin to serve primarily as a store of value, or did he build it for making payments?
To answer this question, I went straight to the source. Thanks to the Satoshi Nakamoto Institute’s archives I was able to read every single thing Satoshi ever posted publicly. Comments from 260 forum threads, 63 emails, and his original source code.
After reviewing all of Satoshi’s writings, I can confidently state that Bitcoin was not purpose-built to first be a store of value. It was built for payments.