As Michael indicated, the text_search table contains a good bit of information about search terms and their results (good for finding searches that yielded no results). You can also look at the ts_search_summary table which lists search terms against various tables and the frequency of the search terms (good for determining which terms are frequently searched). This powers the Top Searches homepage. Between those two tables, you should get all the info you need to target gaps.
In case you wonder about Global Search, in ServiceNow API terms, there is actually no truly global search. The "Global Search" is actually a series of table based full text searches. So if you configure the Global Search to display Knowledge and Incidents, the Global Search actually performs 2 text searches, one on each table. So in the text_search table, when you perform a Global Search, you will see a new entry for each table.
