Export account or periods of a account to excel file
Add the posibility of export account records or partial periods
As the link in an earlier comment currently states: as of GnuCash 3.0 it is possible to export transaction data to CSV. Excel can perfectly import CSV files so this request has now been fulfilled.
To export you can open a register or general ledger. Optionally filter the data to only what you want to export and then select File→Export→Export Active Register to CSV.
If you want to export all transactions in one or more accounts, you can instead use File→Export→Export Transactions to CSV