The best way to seal the sump gasket on an Opel Corsa Gamma that keeps leaking is to follow these steps:

1. Start by removing the old sump gasket. Ensure that the surface where the gasket sits is clean and free from any debris or oil residue. Use a gasket scraper, such as a plastic or nylon tool, to carefully scrape away any remaining gasket material.

2. Inspect the sump for any signs of damage or warping. If you notice any major cracks or deformities, it may be necessary to replace the sump itself.

3. Next, thoroughly clean the mating surfaces of both the sump and the engine block. Use a gasket cleaner or a mild solvent to remove any oil or residue.

4. Apply a thin layer of high-quality gasket sealant to both sides of the new sump gasket. It's important to use a sealant that is specifically designed for automotive applications and compatible with your engine's oil type.

5. Carefully position the new sump gasket onto the sump, ensuring that it lines up with the bolt holes. Start by hand-tightening the bolts to hold the gasket in place.

6. Gradually tighten the sump bolts in a crisscross pattern, working from the center outward. This will help distribute the pressure evenly and prevent any warping or damage to the gasket or sump. Refer to your vehicle's service manual for the recommended torque specifications for the sump bolts.

7. Once all the bolts are tightened to the proper specification, allow the sealant to cure for the recommended time provided by the manufacturer. This is typically a few hours or overnight.

8. After the sealant has cured, clean any excess sealant that may have squeezed out during the tightening process. Use a clean cloth or rag and a mild solvent to gently remove the excess sealant.

9. Finally, start the engine and check for any signs of leakage. If the sump continues to leak, it may be necessary to repeat the sealing process or consider seeking the assistance of a professional mechanic who can diagnose and repair the issue further.

Remember to always follow the vehicle manufacturer's recommendations and guidelines and refer to the specific service manual for your Opel Corsa Gamma to ensure proper procedures are followed.