Account Summary report, which summarizes all incomes and expenses in every currency in the current fiscal year.
This is wrong. Account Summary is formally known as "Trial Balance". It is a snapshot of all accounts on a given date. This means that this report shows all income and expenses since you opened the books, or the last time you closed the books. This could be more than 1 year, or even 10 years of expense.
Basic Financial Statement includes at least two things: Balance Sheet and Income Statement. For GNUCash they can be found under [Reports -> Assets & Liabilities -> Balance Sheet] and [Reports -> Income & Expense -> Income Statement].
Balance Sheet is a snapshot of Assets and Liabilities on a single day. Whereas an Income Statement represents Income and Expense in a period.
This means asking "What is the Balance Sheet from April 1 - June 30" does not make any sense. You may only ask "What is the Balance Sheet on June 30".
As for Income Statement from April 1 - June 30, after going into [Reports -> Income & Expense -> Income Statement], you may use [Options -> General -> Start Date/End Date].
There is no reason to use "Account Summary". The last important Financial Statement is "Cash Flow".
For more information read this.