Well... I don't see that as a problem. Gift tax would be between €1,500 and €3,000 depending on the property value (at €50,000 or €60,000), since the original poster also has a tax allowance of €20,000. For a property value of €50,000-€60,000, that's peanuts. Possibly, a sale with a gift portion might also be possible. In return, the tax office could then impose real estate transfer tax. This should be discussed with a professional (tax advisor/lawyer).
Example: property value €60,000
Sale from grandparents to the original poster of 1/2 property for €10,000
Value of half the property = €30,000
Gift portion €20,000
Allowance €20,000
Gift tax €0
Real estate transfer tax on €10,000 in Brandenburg €600
Gift from grandparents to the original poster's girlfriend of 1/2 property plus €10,000
Value of the gift €40,000
Allowance €200,000
Gift tax €0
If the wedding is planned anyway, it could be brought forward; then the gift tax rate might be lower or the entire property could be given to the granddaughter and half the property in step 2 to the husband as a gift.