I use GnuCash for my own bookkeeping and like it a lot (but not connected to farmOS).
In fact, before farmOS I started a project called Ledger, which aimed to be a GnuCash clone for Drupal. farmOS’s log-based architecture is very much inspired by double-entry bookkeeping. And my long term vision was to eventually include Ledger in farmOS to serve the “bookkeeping” side of things. I’d still love to see that happen, but it has fallen very far down the priority list. Maybe someday… 
(Here’s the old issue I created for that: https://www.drupal.org/project/farm/issues/2622552)
In my mind, farmOS’s long-term trajectory is to eventually include “bookkeeping” alongside the existing “recordkeeping”. And in practice this may mean simply integrating with other software like GnuCash/Quickbooks. But either way, we do need to have some data architecture in farmOS itself to connect things up.
We took a very simple first step in 2017(ish?) by adding Sale and Purchase logs (available in the “Farm Ledger” module in farmOS 1.x). But ultimately, I think all Log types should have the ability to store value information - so that you can truly track costs in a granular way (and therefore relate them back to the assets they reference).
Related to this is time tracking: Additional log "time" features - any interest?
Logs can already have “Quantities” associated with them, which can be used to record “Time” and “Value” measurements. But this is still pretty low-level, and there isn’t any supporting UI/UX code to standardize how it is used. Ultimately we need to develop some agreed-upon conventions (Creating a standard 'interpretation' layer on top of core farmOS schema).
We are in the process of adding two new Quantity types to farmOS 2.x to help with this:
- Price quantities: https://www.drupal.org/project/farm/issues/3196836
- Time quantities: https://www.drupal.org/project/farm/issues/3196844
Hope that all provides some context. I’m excited to see the financial side of things develop in farmOS.