Yesterday I commited a patch allows using GMP for bignum computation in 32-bit sbcl on Intel/AMD processors.
To use it configure using '--with-gmp' option -- you must have gmp installed (including C header files, that is devel package), otherwise build will fail.
Actually, there a chance that the code will work on other 32-bit processors, but I have no way of testing on such processors. Consequently, currently configure will signal error if you try to use '--with-gmp' on something other than 32-bit or 64-bit Intel/AMD processors.
Also '--with-gmp' will fail if you try to use different Lisp: ECL and GCL already use GMP and currently using GMP with Closure CL and Clisp is unsupported (I have code for 64-bit Closure CL on AMD64, but since number of users of FriCAS on top of Closure CL seem to be low (0???), I did not integrate that code in FriCAS build process).
BTW, updated timings for 10^6 digits of e:
native sbcl sbcl + GMP 5.0.1
32-bit 69s 1.35s
64-bit 15s 0.6
As you can see going to 64-bit gives substantial speedup for arithmetic, which in practice may be more significant than speedup due to use of GMP: GMP gives large speedup for large numbers, but for smaller ones effect is smaller. 64-bit arithmetic gives speedups at almost all sizes: huge speedup between 30 and 60 bits (due to use of fixnums instead of bignums), small speedup for slightly more than 60 bits growing to about 4 when having about few hunded bits.