Well, actually Zope was configured to listen to 'localhost' IP since that worked (from outside as well as locally) fine when it runned on my Desktop PC.
Now when i installed it on the gateway PC, I first configured it to listen to my ISP's provided IP, but it didn't run this way, so i renned back & told it to listen to 127.0.0.1 as before...
... until I made a better iptables configuration !
I just tried again :
- tell Zope to listen to my ISP's provided IP,
- try to access it from my desktop PC
and that just runs fine
The problem here was an uncomplete iptables configuration, then a wrong configuration of the web server.
Thank you DaveG for helping me to solve my problem