Hello Xavier,
Your question is a bit unclear to me.
Where do you have a problem?
a) Finding your KSK for your zone?
b) Finding the DNSKEY text box on the Gandi website?
Answer to a):
knotc zone-read <domain> <domain> DNSKEY
Answer to b):
I'm a Gandi customer myself. Gandi requires the DNSKEY (not the DS
record). Note also that Gandi does not support DNSSEC for all TLDs. This
is indicated in the Gandi user interface when managing a domain.
Daniel