Using auto complete handle (+) with formulas

Hi all - sorry I'm new to this so if i'm in the wrong forum, please forgive me...

I have the following formula in a cell C6 : "='Data Dump'!G20"

I want the adjacent cell on the right (C7) to be "='Data Dump'!G21" and C8 to be "='Data Dump'!G22" and on and on.

I'm trying to use the Auto complete handle on the bottom right hand corner of the cell (which looks like a + when the cursor is over it) to drag the formula so that the "Data Dump - G" values increase as I go (g21, g22, g23 and onwards)

However, rather than the numerical part of the cell incrementing, the character (letter) does so instead, so I get H20, I20, J20 and onwards.

Can anyone tell me how to do what I want it to do?

Any assistance would be very gratefully received