SSL Inspection and certificate error

I understand that if I want to do HTTPs inspection I need to enable SSL inspection on fortinet but this gives error on users browser when opening https websites. This is because we need to install fortinet certificate in user PC , once this is done error goes away . 

In guest case senerios where users bring in their own device and we dont have option to install this certificate on each of those devices how would https blocking work ? I believe that we need to instsall the ssl certificate because our certificate is a private generated one , if we purchase a certificate from a known company like https://www.rapidssl.com etc and use that certificate in fortinet and not the default one of fortinet , we might not need to put that certificate in each user PC  because this certificate would be globally trusted . 

Please advise if i am correct.