Well ... GCSE as in the same disc OAA media sell as seen
here.
Contents Include:
General Number Work
Fractions, Percentages, Decimals
Ratio and Proportion
Use of Symbols
Equations
Formulae
Sequences
Using Graphs
Diagrams and Charts
Averages
Probability
Angles
Perimeter, Area, Volume
Circles
Triangles
Other Shapes
Transformations
Inequalities
Simultaneous Equations
Quadratic Equations
Algebraic Fractions
Non-Linear Graphs
Tangents and Areas
Cumulative Frequency
Histograms
Sampling
Standard Deviation
Trigonometry
Vectors
I've not sat any ATPL GS exams yet but I'm told they don't go much deeper than the above. So if you can carry out Trigonometry and Vectors like falling out of bed then I guess it's a head start. Trying to get to grips with a scientific calculator half way through your ATPL course is probably leaving it too late
