General GL codes
Add GL Codes
Go to More -> List Management -> Accounting -> Charts of Accounts (GL Codes)

Click the 'Add' button to add a new GL code.
Click the Ellipsis -> Modify to edit an existing GL code.
Transactions By Account will take you to Journal Entries for the GL code: Read more about Journal Entries

When creating a GL code select the Account Type and give it a name.

You can write notes about this GL code and if desired itemize it by state.

Add GL Code to Tasks on Portal
Go to More -> List Management -> Quotes, Jobs, & Invoices -> Task Options -> Tasks

Double click on a task or select the Ellipsis -> Modify

Select the desired GL code from the dropdown in the General Info table.

Repeat this with every task you would like to attach a GL code to.
Congratulations, you have set up your GL codes!
FAQ:
GL Code for Parts:
Physical Parts and Inventory are always assets in all Accounting softwares. If you took out a loan to buy the inventory, that's a liability.
The GL codes on the Sales Tab of the part and so that GL code is what revenue account it goes to when you mark the part as billable.