I wouldn't call it "atrophy" unless you were already reading and writing Asm regularly.

I have not used FASM so I can't comment on it, but there are quite large differences between assemblers in general. Each is essentially a (non-optimizing) compiler for a different extension language. Writing your own assembler that literally just assembles is trivial, but writing raw assembly instruction by instruction is tedious and not especially practical. If you're serious about writing it, you'll want macros and other forms of metaprogramming; as a programmer, the entire point of your existence is to automate tedious, repetitive tasks, and better assemblers help with that. Programming your own assembler is a natural progression of metaprogramming if you stick with assembly long enough, but first you should learn with using an existing one so you can begin to understand your own workflow well enough to know what tools you want for reducing repetition and where the existing one falls short.

Gdb is an excellent debugger for assembly, even when the original code was written in C

I would automatically type

(gdb) x/4i $pc

when opening a coredump out of habit.

But using nasm, then looking as gas style assembly in the debugger might be a bit more jarring.

What's the better choice?