Hi Molly, as far as I know there is no direct viewer for brushes outside of GIMP.
If you press Ctrl + Shift + B in GIMP, it will bring up the Brushes Dialog where you can see them. Some brushes you can delete outright from the the dialog by clicking the brush then clicking the Trashcan at the bottom. Others, the Trashcan is grayed out and you can't delete them.

I have scaled my Brushes Dialog down so that it is smaller than usual; I wanted a small window for the images. The selected brush has a black border around it.
Ones that can't be deleted are brushes that come bundled with GIMP and some are brushes that have several brushes in one file. Frequently PhotoShop brushes with an abr extension are like that.
If you right-click on a brush with the Trashcan grayed out, there is a dropdown list where you can Copy Brush Location. Click that and Paste it into a text editor like Notepad. It will show the path and name of the brush so that you can browse to the location and delete the brush from there.

If you copy all of the locations of the ones you want to delete first, you can print the list and then browse to the brushes.
Keep in mind, when there is more than one brush in the file, you will be deleting the whole group.