I am not sure if you are looking for a horizontal descendant tree or just alternatives to printing out a descendant tree other than using Gramps itself. I am going to focus on alternatives to built in charting like what would be in Gramps, FamilyTreeMaker, Mac FamilyTree, etc. as well as point out which ones do horizontal trees.
Depending on which platform you are on there are alternative options with each application having its own charting options.
When I was a FamilyTreeMaker user on Windows many years ago I supplemented their lack of charting with Progency Genealogy Software. When I switched to Mac they did not have a Mac version and I found that MacFamilyTree has almost all of their charts built into the core program. It also had several descendant trees/ancestor trees and I really like the double sided ancestor tree.
I am not necessarily recommending one over the other.. just informing of options I am aware of to accomplish your needs.
Mac Family Tree's Horizontal:
One of the chart's in Progency's Software top/down and horizontal.
Web Family Tree I recently discovered and it is a hybrid tree that is editable using GED file and is free at this time.
WebTreePrint can do these:
GedScape has Horizontal and Top/Down Charts