If you don't already know Java, these pages are for you:
3 pages on Groovy Math explain, in depth, the facilities available from both Groovy and Java:
Groovy Integer Math
Groovy BigDecimal Math
Groovy Floating Point Math
Blocks, Closures, and Functions explains the syntax of blocks, closures, and functions.
Java Reflection in Groovy explains how to examine and manipulate objects and their fields and methods when their names aren't known at compile time.