The core activity of Grab Baugeschäft Baudienstleistungen Immobilien AG encompasses three main areas:
1. **Construction Execution**: The company offers construction execution services, including planning, building, and execution of construction projects.
2. **Construction Services**: Grab provides specialized construction services such as drilling and cutting work.
3. **Real Estate**: The company is also active in the real estate sector and offers properties for sale and rent.
Overall, the core activity of the company can be described as comprehensive services in the construction and real estate sector.