> Nixos on my machine uses the abi n32 (long and void* have 32 bits, long long
> 64). I would like to switch to abi 64 (long with 64 bits); we might be the
> first distribution supporting this mode.

I think that the above contradicts the following.

> In the debian distribution also installed on the machine,
> there is a separate lib64 directory, and it is enough to pass the flag "-mabi=
> 64" to gcc.

Is it supported in Debian?

Also, what about compatibility?  Are there any downsides?

"But an ABI can be chosen explicitly to make GMP compatible with other
libraries, or particular application requirements." [1]

I'll give it a try if you provide detailed instructions.

[1] http://gmplib.org/manual/ABI-and-ISA.html