For a GUI solution you could use [Audacity][1] but with the proviso that it will work on one file at a time by default.

 - Free, Gratis & Open Source
 - Cross Platform - builds available for Windows, OS-X & Linux
 - Supports multiple formats including FLAC
 - You could write a [plug-in][2] using Batch Chains to Normalise all of the selected files or a directory worth of files.
 - GUI based
 - Can do a lot more of course.

For anybody who is not so averse to the command line see the SU question and answer [here][3] for a mechanism that would let you type `ffmpeg-normalize *.flac` in a directory full of such files - of course you could always add a file/directory select GUI to the solution given there will a little effort.

Update
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In fact there is an [chains example][4] Example 1 that fits you case well.

Constructing a Chain
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To construct the chain that you need:

 1. start Audacity with nothing loaded
 1. select `File->Edit Chains...` and you get a window like:[![enter image description here][5]][5]
 1. On the bottom left select `Add` and you will be prompted for the name, *give it one that makes sense to you and click `OK`*:
[![enter image description here][6]][6]
 1. Click `Insert` and you will get the Select Command pop-up, Pick `Normalize` and if you feel the need play with the parameters by selecting `Edit Parameters`, *the defaults look quite sensible to me* then click `OK`: 
[![enter image description here][7]][7]
 1. Make sure `-END-` is still selected and then Click `Insert` again and pick your export, *presumably ExportFLAC*:
[![enter image description here][8]][8]
 1. Your "Edit Chains" window should now look something like the following, if it does click `OK`:
[![enter image description here][9]][9]

Using you new chain
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 1. Open Audacity with no files loaded *if it is not already open* if any files are loaded select `File->Close`
 1. Select `File->Apply Chains...` and on the dialog select your new chain and select `Apply to Files` and then in the file select dialog navigate to the files you wish to convert and select those files.

[![enter image description here][10]][10]

 1. Depending on the number of files that you selected do one of: watch it in action, go and have a cup of tea, go and have a meal, go to bed for the night.
 1. Find your new files in a sub-directory of the original called `cleaned`

Important Note
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The **good news** is that your original files are not overwritten so they are safe, the **bad news** is that your original files are not overwritten so the disk space taken **doubles!** I would recommend running on smallish directories of your music/sounds at a time, checking the results to make **sure** that you are happy, then moving the originals to backup media.


  [1]: http://www.audacityteam.org/download/
  [2]: http://wiki.audacityteam.org/wiki/Creating_your_own_Plug-in
  [3]: http://superuser.com/questions/323119/how-can-i-normalize-audio-using-ffmpeg
  [4]: http://manual.audacityteam.org/o/man/chains_for_batch_processing_and_effects_automation.html##Example_1:_LoudMP3
  [5]: https://i.sstatic.net/XLe8U.png
  [6]: https://i.sstatic.net/1g4rF.png
  [7]: https://i.sstatic.net/sOWtz.png
  [8]: https://i.sstatic.net/axKQN.png
  [9]: https://i.sstatic.net/qQhXV.png
  [10]: https://i.sstatic.net/4HaUM.png