The gap is produced due to the fact that the group footer section is printed at the end of each group of records and right after the detail section. The page footer section is printed at the end of every page. On different pages, you have different height of the detail section, so there is a gap where the height is smaller.
As for the second requirement, please, try using the PageExec(itemName, aggregateFunction) page scope related function which executes the specified aggregate function in the given scope. As the name suggests, it can be used in page header or page footer of the report. In order to specify the scope in which the aggregate would be evaluated, you need to specify a report item as a parameter of the function.
For example, you can specify the TextBox item which contains the name of the first record as the itemName and the First(Fields.TestColumn) aggregate function. The same applies for the last record. I attached a sample report to demonstrate the scenario. It uses AdventureWorks test database to retrieve a sample data.
For more information on the PageExec function, please refer to the Data Functions help article.
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