Workaround
  1. Generate a "Capital gains" report for custom dates: Jan 1st - Jun 30th (How to generate reports for custom dates)
  2. Open the file in Google Sheets
  3. Select the table and click Insert > Pivot table (see screenshot)
  4. Choose "New Sheet" in the popup
  5. Drag and drop "Asset" to "Rows", then "Amount", "Proceeds", "Cost", "Gain/Loss" to "Values" (see screenshot)
  6. Use the sums in the table to fill the form
  7. Repeat steps 1-6 for a report generated for Jul 1st - Dec 31st