Bob Phillips wrote...
Try this, as dynamic as it gets (I hope -;))

On the summary sheet,

cell A1: enter the first data sheet name
Cell B1:
=SUMIF(OFFSET(INDIRECT("'"&A1&"'!A1"),,,COUNTA(IN DIRECT("'"&A1&"'!A:A"))),"G
1",OFFSET(INDIRECT("'"&A1&"'!B1"),,,COUNTA(INDIRE CT("'"&A1&"'!A:A"))))
....

Why OFFSET(INDIRECT(.))? An alternative,

=SUMIF(INDIRECT("'"&A1&"'!A1:A"&COUNTA(INDIRECT("' "&A1&"'!A:A"))),
"G1",INDIRECT("'"&A1&"'!B1:B"&COUNTA(INDIRECT("'"& A1&"'!A:A"))))

That said, I doubt this would recalc any quicker than

=SUMIF(INDIRECT("'"&A1&"'!A:A"),"G1",INDIRECT("'"& A1&"'!B:B"))