As Tiffany said, at this time, Goodbudget isn’t really designed to handle investments or other types of mostly off-budget accounts with fluctuating values.
However, off the record, I track my retirement accounts in Goodbudget, and I do so by using Debt Accounts…but in reverse. That way the money stays off-budget and you don’t have to deal with Unallocated and updating both an Envelope and Account when there are changes. It’s still not perfect, because you have to cue the fluctuations manually (I do so quarterly…or less, because I forget), but it should work.
(The reason I say this is off the record is because I want to be clear I’m saying this as a Goodbudget user and not from my position as a staff member! This is definitely a “hack” or workaround, and isn’t intended design, so I’m obliged to say that since it’s not supported, other updates we might make very well might prevent this as a future option. But for now, that’s what I do.)