Gandi updates our zone every 20 minutes.
This means that if you have just added an entry in your zone file, you will then have between 0 and 20 minutes to wait before your addition is reflected in the zone.
Why 20 minutes and not every minute? This time is necessary because we regenerate the entire zone each time, and it takes a while to replicate it on all the servers.
Gandi only saves the 5 most recent zone files (or the active one, and the 4 most recent ones).
The handle you are logged in under does not have sufficient rights to access this zone.
To edit a zone, you need to be logged in under the zonefile' owner. Otherwise you need to first make a copy of the zone so that you own the copy, then you can make changes.
You can see directly from the domain's control panel whether or not the handle you are logged in under can edit a zone.
The solution is to proceed by creating a copy of the zone and then following the instructions here to go through with the normal zonefile edit process.
If you have a message saying that the dns was refused by the registry because the zonecheck failed, please contact your DNS provider (usually your webhost). More about this here.
Once they fixed their DNS to work for the domain, and it passes the zonecheck, then relaunch the DNS change at Gandi from your domain's control panel.
There is no reason to contact Gandi's support for this as it is entirely between the DNS provider and the registry.