Building plot as a gift / not married

Everything is also included with us. If she dies, I as the property owner must pay her parents, and vice versa, my parents would have to pay her, and the house would pass into their ownership. Upon marriage, the statutory inheritance succession then replaces this arrangement.

Death :p. You are funny. The probability that she leaves you or you break up is much higher than the mentioned death. Don’t worry about it. Statistically, you will die earlier than her anyway. The topic of breakup is what must be clarified in any case.

Nonsense. Statistics have virtually nothing to do with the individual case and, after all, not so few young people also die.
No relationship lasts forever and there are exactly 4 ways it can end: breakup, one dies, the other dies, both die "in joint danger" (= "simultaneously"). There is nothing more. It’s not too much to ask to think through each of these possibilities.

And yes, of course inheritance tax applies if someone other than a legal heir leaves you a house by inheritance law.

And yes, of course inheritance tax applies if a "other person" according to inheritance law bequeaths you a house.

To avoid taxes in the case of protection (aka life insurance), at least cross insurance can be done. (Insured 1 + Policyholder 2 and Insured 2 + Policyholder 1; Insured 1 = Policyholder 1; Insured 2 = Policyholder 2)

Is it also planned to build on the property right away? And if so, will you pay off the house half and half?

Then one possibility might be that you are listed in the land register proportionally. For example:
Property value €100,000, your house another €400,000. Then you could be listed in the land register with 2/5 and your girlfriend with 3/5.

That would be the option if your girlfriend keeps her property value-wise.

Perhaps there are also other possibilities for splitting in order to use the exemption allowance multiple times.

Example 1
Your girlfriend receives 3/4 as a gift (due to a high exemption allowance). You receive 1/4 as a gift. In step 2, your girlfriend gives you 1/4 as a gift.
Disadvantage: costs for multiple notary contracts.

Example 2
It might also work if the property belongs to both parents-in-law, with the mother-in-law gifting one part and the father-in-law gifting another part.

In any case, consult a tax advisor.