This might a good time to remind everyone that every single program compiled by a standard GHC is linked against an LGPL library (the Gnu multi-precision integer library) - unless you take care first to build your own
This is less relevant though, because gmp is not a Haskell library so linking against it doesn't pull significant chunks of its source code into your program. Indeed, many GHC distributions dynamic-link against it, minimizing the legal concerns; and IIRC the gmp license explicitly allows the use GHC makes of it as long as the gmp symbols are not re-exported as part of its API (which they aren't).