I still miss a good debugging GUI with useful data visualizations. Sometimes staring at pointers and (multidimensional) arrays of numbers just isn't enough.
For data visualizations I use gdb's Python API. For simple things that can be just adding pretty printers, for hairy things it can be a custom command that prints extra information or dumps it to a graphviz script.
https://sourceware.org/gdb/onlinedocs/gdb/Python-API.html