No I don't mean something like this. Actually, a text such as many cars have a GPS that help drivers to travel etc is grammatically correct.
Of course the mention on above, it is not a rule in English either in Greek.
One more example of redundancy (in my native) is the following: Can you repeat this again? Because you speak very fast.
"Many cars have a GPS that help drivers to travel etc" is a very poor English sentence, not "grammatically correct" as you stated. For a start, it should be "helps" because it refers to "a GPS" (singular). Saying "drivers to travel" is unnatural too. We might say "helps people to drive ..."
"Of course the mention on above" is incorrect English.
"It is not a rule in English either in Greek" is incorrect English.
"(in my native)" should read "(in my native language)".
"Can you repeat this again? Because you speak very fast" is incorrect English. The second part is a fragment, not a complete sentence. You will hear "Please repeat that again?" from a lot of native speakers, grammatically correct or not.
I notice that you have not started your post with "NOT A TEACHER". There are multiple errors in your post and it is unfair on other learners to read your response, thinking that you are advising them correctly when you are not. Please think very carefully before responding to other learners' posts and consider whether or not you are 100% sure of your advice and the accuracy of your English before you do. Add "NOT A TEACHER" at the start of every response.