I looked it up, and it seems that one issue is that Google, as a DMA gatekeeper, is supposed to provide anonymous data to other search services, but in practice, important parts of the actual data have been removed. The other issue is that competing AI services (not just Gemini) should also be allowed to run integrated app search.
And supposedly, "steering must be free". Or at least that's what they told Apple when fining €500M, I think today they both charge up to about 20% commission if apps use third party payments.
Reading about it, both points seem valid to me.