Refer to: Gnu - Global - Help - How to find the definition effectively
To put the story even shorter, the GNU GLOBAL will not treat C struct as definition, but as other symbol' . This means you may only find a struct withglobal -s`, which will give you lots of irrelevant results.
This feature is in the GLOBAL plans for the future already, which means it might be implemented sometime in the future. For now, it’s better to use GLOBAL to trace how the program runs, the relationship between functions, rather then finding the semantic details. For the semantic side, the ctags or etags is still my first choice.