Thelma:
...
> https://packages.gentoo.org/packages/app-office/glabels
> 
> It seems that version 3.99 is available upstream, while the latest
> version in the Gentoo tree is 3.4.1.

There is two glabels, the "old" 3-series, and the newer "qt"
4-series, you can get them with:

git clone https://github.com/jimevins/glabels.git
git clone https://github.com/jimevins/glabels-qt.git

///

BTW, reading
 glabels-qt/docs/BUILD-INSTRUCTIONS-LINUX.md 
I see the following excerp:

 <<
As of version 0.99, GNU Barcode no longer installs its library.  So install 0.98 from source:
```
wget https://ftp.gnu.org/gnu/barcode/barcode-0.98.tar.gz
tar xzf barcode-0.98.tar.gz
cd barcode-0.98/
./configure && make
sudo make install
```
 >>

Doing just that and then reemerging glabels, gives me the missing gnu 
barcode backend.

Regards,
/Karl Hammar