You can change what is taken from the certificate for matching it against the user base (LDAP or local).
Before R80.x it was a bit of a pain to configure through GuiDBedit, but since R80.10 you can select it when configuration Multiple Login Options:
https://sc1.checkpoint.com/documents/R81/WebAdminGuides/EN/CP_R81_RemoteAccessVPN_AdminGuide/Topics-...
See chapter: Certificate Parsing
Besides the "Fetch username from" setting as described, you will have to match the "search LDAP for", so it can find it.
So you can even go for CN, SAN.email, SAN.UPN, etc...
Btw. I configured this on R77.10 already but not that comfortable 😉