BigInteger

There are several (free) Pascal libraries for BigInteger (and sometimes Big Decimals like 1.23) functionality. Note: the C-based GMP library can also be used in Free Pascal, it's not listed here.

TForge

Homepage: https://github.com/Alexey-T/TForge-Pascal

Modern cryptographic library for Delphi and Lazarus/FPC implemented as a set of runtime packages. Project development was stopped, the last found snapshot v0.76 was put to GitHub.

Big Decimal Math

Homepage: https://benibela.de/sources_en.html#bigdecimalmath

This unit provides a BigDecimal-Type which stores arbitrary precision (BCD) decimal numbers and can be used like any native numeric type.

FNX

Homepage: https://www.ellipsa.eu/public/fnx/fnx.html

Library of multi-precision numbers (currently, only big integers) written for Free Pascal and Linux. The assembler code that can be enabled with a compiler directive, is for x86-64 processors only.

MPArith

Homepage: https://github.com/Alexey-T/Wolfgang_Ehrhardt_codes/tree/master/Misc/MPArith

Project development was stopped in about 2018. The last found snapshot was put to GitHub.

DelphiBigNumbers

Homepage: https://github.com/rvelthuis/DelphiBigNumbers

BigInteger and BigDecimal for Delphi. Code can be adapted for Free Pascal with a little effort.

BigIntegers for Delphi

Homepage: http://www.rvelthuis.de/programs/bigintegers.html

But it not for FPC, only for Delphi.