GDP is not "imaginary" or "not tangible" anymore than an accounting statement of profit and loss is imaginary. It is a very real thing but you have some subtle approaches about how you measure things like depreciation or what accounting rules you follow. The fact that a lot of people don't understand this stuff doesn't mean it is arbitrary or has no meaning. There are also huge teams of accountants that spend tens of thousands of hours getting these statistics, and their work is much maligned by the smarmy internet posters who think they know better at how to determine profit and loss.
GDP is just national value added, and was developed in the early 20th century and broadly used during the Great Depression to measure economic activity. It is good at what it does. People have taken it to measure military power, wealth, success, happiness, capability to build a specific thing, fertility, etc. That is the fault of people not understanding what GDP is or what it was meant to measure, it's not the fault of the metric itself, or the carefully developed system of accounting conventions that were developed over a hundred years of refinement.
I really think there should be mandatory accountancy training in universal education to get people to understand this stuff. The millions of hours saved in online arguments alone would make this a good investment.
Having said that, the valuation of tech companies is based on market sentiment, not accountancy rules. I don't pretend to understand how many of the companies on the equities exchanges are valued, they all seem to be too pricey for me. That, too, has nothing to do with GDP.