when using the GAP tool, allow to enter the space size.
When i have a collage of many images and I want to have all the spacing between the images the same exact amount, entering a desired size will be a huge help.
Rob Hutchings commented
As above but with some meat on the bones to describe the expected behaviour:
I should be able to double click on the tool icon to bring up a options window to set the size of the gap I want. Even more useful would be the ability to pre-select a number of objects and then single click with the Gap Tool to resize the gap between the pre-selected objects to the predetermined value.