Not too familiar with the GCSE, but google tells me that you can expect mechanical, electrical/electronic, pneumatics, hydraulics, design, communication, and computer engineering. Sounds like a great subject to take!
As for engineering in general, you can expect to be dealing with maths, science, design, and communication, while working alone and in groups. You’ll be creating new things, carrying out experiments, developing ideas and improving the way things are done.
If there was an engineering GCSE, I would expect it to be based on Science, Technology and Maths skills, but also the ability to problem solve, and think outside the box. Team working and the ability to write technical reports would feature pretty highly, as being able to communicate your ideas is an important part of engineering.