If you ask some company to create some domain for you and several e-mail account with that domain and forward that information to your gmail account...I do not really understand, ANY web developer can check all mails from any client, or Am I missing someting?
In a way your question is funny: you fear that the domain provider might intercept and read your mail when it gets forwarded to your gmail account but you don't fear that somebody is reading the mail at the forwarded account itself, i.e. the mails which are stored at google's servers outside of your control. The mail can be read in both cases and it will at least be done if these providers get a warrant because law enforcement needs to get access to information in thus mails.
If you make decisions to give somebody else such level of control over your data you have to ask yourself how much you trust this company. This is the same question you have to ask yourself with your telecom provider, because both land lines and mobile calls can be intercepted by the provider and this will be done if law enforcement requires it. And the same is true with your internet access.
Thus in your specific case you need to trust google to not read your mails or at least don't use any information in these mails against you. And you have to trust your domain provider to not read the mails when forwarding and also to set up the interface for forwarding in a way which can not be hacked by someone - because then the mail could get just be intercepted some yet another party. And there is probably more trust involved.
How do you forbids that the developer company/reseller can check all your mails?
The only way to be sure that no third party is reading your mails is to encrypt them. This does not mean TLS which does only hop-by-hop encryption between mail servers but the mail is available in plain text at the mail server itself. Instead you need to use end-to-end encryption like PGP or S/MIME. But of course end-to-end only works if sender and recipient use this kind of encryption. And meta data like sender, recipient, time and size of the mail are still visible even if the mail content itself is encrypted.
