: to alter or rearrange especially by manipulating - jigger an election district
"Jigger." Merriam-Webster.com. Merriam-Webster, n.d. Web. 13 July 2018.
More idiomatically - rejigger. It's not specific to removing gaps, but accommodative to that action. It would normally need a bit of lexical help to get that specific point across.
: alter, rearrange
"Rejigger." Merriam-Webster.com. Merriam-Webster, n.d. Web. 13 July 2018.
Interestingly, MW identifies the first as informal, but gives the second a pass.
Grammarist has the following to say - "Both rejig and rejigger mean to rearrange an object. Rejigger is the preferred form in the US, and outside the US the standard is rejig."
WordReference.com has an additional usage note - "to change or rearrange in a new or different way, esp. by the use of techniques not always considered ethical. And they do mark it as informal.
This may often be the context in journalistic use, but in my opinion, isn't associated with the word itself, at least not in my dialect where it is a common, everyday word.
As mentioned above, (re)pack is sometimes more idiomatic. For instance, the hotel desk manager is responsible for "repacking the grid", meaning shuffling room assignments around to remove small vacancies and free up larger blocks of vacancies - repacking in order to remove gaps in occupancy.
Most other options, such as fix or tidy imply more than just adjusting the position of things.