opel astra 1.8 ecotectect motor does not have oil cooler . my oil and water keep on mixing in the water bottel please i need your advice

There are only two possible places where oil and water can come in contact: Either the oil cooler or the engine. Since you don't have an oil cooler there is only the engine left. Your statement, that the oil and water doesn't mix in the engine, strikes me as odd. How else should the oil reach the water, if not in the engine?

Since you stated that the gasket got replaced, I would go back to be shop that did it, to let them fix that, as most likely they didn't do it right (Didn't skim the head?).