Creation and use of filters is documented in the product documentation in the Gosu Reference Guide under the path "Querying and connecting to databases ==> Query builder APIs ==> Working with results ==> Filtering results with standard query filters". Here's a link to the doc but you'll need a GW Community login to access it.

https://docs.guidewire.com/is/gosu/gosu/topics/querybuilderapi/c_ba2592202.html

You can add your filter to the enhancement but in any case you'll need to add a filter option on the target screen so you might just define it inline. The hard work is in defining the clause you want to add to the query and that can be complicated. I'd suggest using the scratchpad to develop and test your filter clause. Details of adding query clauses of various complexities are in the same area of the documentation. I'm not certain you can express your logic entirely as you've described it so you might need to consider the best way to express that logic.

Good luck!