В документации сказано:
#  zip: tree_path [options] 
#  zip?: tree_path [options] 
...
#    suffix=suffix      - override default output file name suffix.

Либо документация не полная, либо suffix= не работает.
Вот тест:

$ find . -type f \! -wholename '*/.git/*'
./.gear-rules
./firefox-switch-locales.spec
./switch-locales/chrome/content/switch-tbird.xul
./switch-locales/chrome/content/switch.js
./switch-locales/chrome/content/switch.xul
./switch-locales/chrome.manifest
./switch-locales/install.rdf
./switch-locales-2.0.xpi

$ cat ./.gear-rules
#copy: *.xpi
zip: switch-locales suffix=.xpi

$ gear -v --commit --export-dir=/tmp/zzz
Created commit 01696302541d274eb7ac1361066de952ba2ff223
 7 files changed, 159 insertions(+), 2 deletions(-)
 create mode 100644 switch-locales/chrome.manifest
 create mode 100644 switch-locales/chrome/content/switch-tbird.xul
 create mode 100644 switch-locales/chrome/content/switch.js
 create mode 100644 switch-locales/chrome/content/switch.xul
 create mode 100644 switch-locales/install.rdf
gear: Temporarily committed local changes.
gear: Extracted "switch-locales-2.0.zip" archive.
gear: Extracted "firefox-switch-locales.spec" specfile.
gear: Reverted temporary commit.

$ find /tmp/zzz        
/tmp/zzz
/tmp/zzz/firefox-switch-locales.spec
/tmp/zzz/switch-locales-2.0.zip