Watashi ha geemu o asobu ga suki desu.
→Watashi wa geemu de asobu noga suki desu.
or Watashi wa geemu ga suki desu.
You don't have to say "asobu",because games are asobi,
But you better say Watashi wa (TV,PC,sport・・・etc.)geemu ga suki desu.
Watashi ha benkyou o nihongo ga suki desu.
→ Watashi wa nihongo no benkyou ga suki desu.
Watashi wa nihongo wo benkyou surukoto ga suki desu.
When someone ask you"Nani ga suki desuka?"
Then, you can say"Nihongo no benkyou ga suki desu."(natural)
"Nihongo wo benkyou surukoto ga sukidesu"(sound like foreigners saying)
No need to say "Watashi wa"