if the protocol allows data transfer, then it also allows distribution of ads.

You do not know what you are talking about.

Ads are almost never displayed by choice of the reader. People have limited resources of attention, they know attention is precious, and they are not fond of things leeching it.

To see an ad in Geminispace, you would need to deliberately open a page with an ad. This is by implementation. There are no embedded ads. There is also no support for tracking in the Gemini protocol, and no JavaScript and such. You don't need ad-infested libraries, the Gemini space is not centralized so that big corporations can't inject ads into it, and running a Gemini server is so cheap that you don't need ads to pay it - costing a few dozens Euro cents per month, if you count the cost of a Raspberry Pi B and perhaps 9 kWh of electricity per year.

But to make it more concrete and practical:

Here is a Gemini-to-HTTP Gateway which shows a page which lists, in section 2.2.1, Gemini aggregators. As an entry point to geminispace:

https://portal.mozz.us/gemini/geminiprotocol.net/docs/faq-section-2.gmi

You can start there and search.

Gemini has maybe ten thousand pages.

Now, show me a single Gemini page with an ad.