The root word is "gamit" ... (gamit is also a noun ... gamit = thing)
There are two verb forms derived from that root:
gumamit - to use (AF) ginamit - - to use something (OF)
gumagamit / ginagamit are both the "uncompleted" (present) tense of the verb.
gumamit is an UM Verb (gUMamit)... it is "actor focussed" (AF) .... this means the actor is in the "ang case", whereas the object is in the "ng case"
Gumagamit ako ng sasakyan (I am using the car) (ako is an ang pronoun, wheras sasakyan is introduced as an object with ng) The emphasis or focus is on the ACTOR .... I am using the car, not my brother !
ginamit is an IN Verb (gINamit) ... it is "object focussed" (OF) .... this means the object is in the "ang case", whereas the actor is in the "ng case"
Ginagamit ko ang sasakyan (I am using the car - the car is used by me) (ko is a ng pronoun, whereas sasakyan is introduced by ang) The emphasis is on the OBJECT ... I am using the car, not the bicycle !
To some degree "gumagamit" corresponds to active voice in English, while "ginagamit" can be translated using passive voice. But this is not the same .... we have to employ the concept of "focus" instead, which is a very important concept in phil grammar.
Just to illustrate and consolidate what Jimmy329 and Bituingmaykinang have already mentioned, let's restate the 2 sentences given by Jimmy329 by using the subject-predicate sentence structure.
1. "Gumagamit ako ng sasakyán" will become:
AKO ay GUMAGAMIT ng sasakyan (I use/am using a vehicle)
The subject above is the Actor "Ako (I)" and that's the focus. That's why we should use "gumagamit", which is an actor-focus verb.
2. "Ginagamit ko ang sasakyán" will become:
Ang SASAKYAN ay GINAGAMIT ko (The vehicle is used/being used by me)
The subject now is the Object "sasakyan (vehicle)" and that's the focus. We have to use the verb "ginagamit", which is an object-focus verb.
Note: The word "ay" is used when the sentence is stated in the subject-predicate form. You would rarely see this word in short or simple Tagalog sentences because we normally begin them with the verb as indicated in the original examples given by Jimmy329.