Thanks all for suggestions and help
Looking deeper in the custom gateway plugin I discover the attempt made by the developer to recognise the an existing user after the payment successfully goes through the bank. But I think is not the right way to do that, and it doesn’t work either.
Anyway I would like to point out that s2member is able to popolate the form with the the right user values if the user is logged in.
Anyway, to minimise the effort we choose to force users to free subscribe to the website, given the fact wp is able to check existing emails/username and let users to buy membership only if they are logged in.
The only drawback is new user are request make an additional step that could be a bit confusing and frustrating for someone.
The ideal would be check email/users exist, hope this feature will be done.
A little of topic:
is s2member still support?
OptimizeMember has a larger support? Could someone give me a link to investigate?