Gardener only mentions net prices

Because it is not [DSVGO] compliant?:p

And of course I asked whether it was gross or net.

Then everything is fine. Remind him of the conversation, the WhatsApp message, or whatever, and then it should be fine as long as he doesn't want to cheat.

Because it is not GDPR compliant?:p

Maybe for that reason too. But mainly because he can then prove in writing that specific amounts were communicated for specific services. Since is an end consumer, these had to be communicated as gross amounts. If nothing is stated, Alessandro as an end consumer can assume that these are gross amounts.

Otherwise, without written documentation, it would be just statement against statement.

Just to make sure I understand correctly: everything is okay with the main contract. Offered gross and billed correctly? The 6000 euros VAT then only come from the additional things agreed upon via WhatsApp or however else? That means we're talking about almost 40 k€ of additional services to the main contract?

Chapeau.
Whoever has it, has it :p

Well, the biggest chunk consisted of curbs and L-stones, as well as additional paving of the sidewalks in front of the house.