BigInteger
There are several (free) Pascal libraries for BigInteger (and sometimes Big Decimals like 1.23) functionality:
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
It even has 2 Lazarus demo projects.
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.