Gardener only mentions net prices

then my assumption is confirmed that a craftsman may only quote or state gross prices! Thank you

For us, it was the case that the landscape gardener always spoke in net prices. But the offer was in net/gross. During the construction "the wall is getting longer, that's 1000 more" was for me in Excel then 1000+19% VAT.

is not allowed. Period, full stop, done! Just imagine if the [GU] would only talk about net prices...

What can I do?

Check the invoice to see if it is otherwise correct. Then complain that it is not correct and that the VAT was mistakenly added instead of just being stated. Then pay on time, reduced by a retention of ten percent to secure the prompt receipt of the corrected invoice, and explain this when making the complaint. Upon receipt of the corrected invoice, I would then start the deadline for payment of the retention. Don’t forget the discount, if agreed upon ;-)

Let me guess, you don't have a written offer or anything that he signed?

If I had an offer for the total costs, this thread wouldn’t be necessary ;)
But:
I have an offer. But you know how it is, especially with the garden. Enlarging the paved area a bit here, running an underground cable over there, changing or expanding the planting, etc. etc.
If I had waited for an offer every time, the garden wouldn’t have been finished in 2022.
Exactly these additional services were agreed upon verbally.