No local search

Currently, there is no on-site mechanism to search the GnuCash mailing lists. Instead, you can search the lists using one of these external sites:

These links prepopulate the search form with the list you want to search.

If for some reason that didn't happen (e.g. if JavaScript is disabled):

For Google
In the search bar, type site:lists.gnucash.org/pipermail/list-name to search a specific list, or site:lists.gnucash.org to search them all
For Nabble
By default, all of the lists will be searched. The individual lists can be found at the Sub-forums link