GIMP Version: 2.8.0
Operating System: Windows
OS Version: XP - SP3
GIMP Experience: Beginner Level

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You can either click on that triangle in the brushes dialog box>Add Tabs>Tool Options or go to Windows>Dockable Dialogs>Tool Options.

edit: lost the plot,became overly complicated
try: find the sessionrc file in the folder C:\Users\"yourname"\.gimp-2.8 Rename it to maybe xsessionrc.
Start up gimp and it should be the default 3 pane mode. Change to single window mode. Set it up the way you want and save tool options, windows positions etc in the preferences menu.
Then you can look at the brush options which are a lot more than just spacing.
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You can get your brushes or any other dialogue back from the Windows 'Dockable' menu.
It is worth noting that the Gimp interface is readily customisable by adding tabs to either the tools or options menus, as previous post.
Also worth a mention, when you have Gimp set up to your liking, go into Edit -> Preferences -> Tool options, click the 'Save Tool Options' now and untick the 'Save Tool Options On Exit'.

Next
Might-or-might-not do what you want but there are a host of brush dynamics setting to consider.
First of all to get the options enabled, either copy one or all of the dynamics presets, 'something.gdyn' from the global folder to local folder. Doubles up the presets but the ones 'something#1' are editable and it is safe.
or
Enable the 'writeable' tick box in the edit-> preference-> folders. This also tells you the location of the folders. Not so safe.

Then there are options that you can play around with for any particular brush, the mapping matrix and the dynamics curves for any particular property. Its not M$ and 'given-to-you-on-a-plate', lots to learn.

Keep trying.