I confess the align tool is not one of my favourites. If you have a mixture of icon sizes it can be easier to set up a grid and manually move around.
However, using the align tool.
Align select a base icon
Rubber band the rest
Align horizontally
Set an offset (typically the size of the largest icon)
Distribute by side (left or right)
Image -> Fit Canvas to layers
..and if any small icons try: Layer -> New from Visible then Image -> Zealous crop
example using Gimp 2.8 : https://i.imgur.com/GZ3AI3G.mp4
Edit: An alternative, (I see from an old post you are using 64 bit Windows) You can get a Gimp 2.8 gimp_gmic_qt plugin here: https://gmic.eu/files/windows/
Then using the g'mic Arrays &Tiles > Montage filter
Montage type - Horizontal
Merging mode - Aligned
Centering - 0.5
Input layers - All
Output Mode - New Image
Example: https://i.imgur.com/OADSlR6.mp4
However, using the align tool.
Align select a base icon
Rubber band the rest
Align horizontally
Set an offset (typically the size of the largest icon)
Distribute by side (left or right)
Image -> Fit Canvas to layers
..and if any small icons try: Layer -> New from Visible then Image -> Zealous crop
example using Gimp 2.8 : https://i.imgur.com/GZ3AI3G.mp4
Edit: An alternative, (I see from an old post you are using 64 bit Windows) You can get a Gimp 2.8 gimp_gmic_qt plugin here: https://gmic.eu/files/windows/
Then using the g'mic Arrays &Tiles > Montage filter
Montage type - Horizontal
Merging mode - Aligned
Centering - 0.5
Input layers - All
Output Mode - New Image
Example: https://i.imgur.com/OADSlR6.mp4