The differences between the the GCSE and IGCSE at Foundation level are fairly minimal. If you need to know them, they are available from your specific examination board.
The differences here are more significant.
For Higher Tier, the topics/skills that are in GCSE but not i-GCSE are:
geometric sequences
nth term of a quadratic sequence
exponential functions and exponential growth
equation of a circle and of the tangent to a circle at a given point
finding the area under a graph
using iteration to find approximate solution to an equation
systematic listing strategies: product rule of counting
proofs of circle theorems
frustum of a cone
box plots (box-and-whisker diagrams)
enlargement with a negative scale factor
and in IGCSE but not GCSE:
intersecting chords theorem (in circle theorems)
modulus of a vector
sum of the first n terms of an arithmetic series
understanding domain and range
differentiation with positive and negative (but not fractional) indices
using differentiation to find gradients and stationary points on a curve
optimisation problems using differentiation
applying differentiation to linear kinematics problems (displacement, velocity, acceleration)