kazoku no atumari de ha, sohu no hanasi wo kikuno ga tanosimi desu.

で marks the setting of an action, so 家族の集まりで means at a family gathering.

When it becomes では, the は adds a topic or contrast nuance. So 家族の集まりでは feels like:

• At family gatherings, ...
• When it comes to family gatherings, ...
• At least at family gatherings, ...

In this sentence, では sets family gatherings up as the context being talked about. It is not always strongly contrastive, but it can sound slightly like the speaker is focusing specifically on that situation.

Here, の links two nouns.

• 家族 = family
• 集まり = gathering

So 家族の集まり means:

• a family gathering
• literally, a gathering of the family

This use of の is very common in Japanese. It is not only possession like my book; it can also show category, relationship, or description.

Examples:

• 日本の文化 = Japanese culture
• 学校の先生 = school teacher
• 春の雨 = spring rain

祖父の話 most naturally means:

• my grandfather's stories
• what my grandfather talks about
• my grandfather speaking

It does not usually mean talking to my grandfather.

That is because 話 here is a noun meaning story, talk, or what someone says. The の shows that the talk/stories belong to or come from the grandfather.

So:

• 祖父の話を聞く = listen to my grandfather's stories / hear my grandfather talk

If you wanted to say talk with my grandfather, you would use a different expression, such as:

• 祖父と話す = talk with my grandfather

Because 聞く takes a direct object marked by を.

Here the thing being heard/listened to is:

• 祖父の話 = my grandfather's stories / what my grandfather says

So:

• 祖父の話を聞く = listen to / hear my grandfather's stories

This is the normal pattern:

• 音楽を聞く = listen to music
• 先生の話を聞く = listen to the teacher's talk

The の turns the verb phrase into a noun-like idea. This is called nominalization.

• 聞く = to listen
• 祖父の話を聞く = to listen to my grandfather's stories
• 祖父の話を聞くの = listening to my grandfather's stories

This allows the whole action to become the subject of the sentence.

So the structure is:

• 祖父の話を聞くの = listening to my grandfather's stories
• が楽しみです = is something I look forward to / is enjoyable

Together:

• 祖父の話を聞くのが楽しみです = I look forward to listening to my grandfather's stories

Yes, 祖父の話を聞くことが楽しみです is grammatically possible.

But の sounds more natural in many everyday sentences like this, especially when talking about an activity in a direct, personal way.

Very roughly:

• の often feels more immediate and conversational
• こと can sound a little more formal or abstract

So in this sentence, 聞くのが楽しみです is the more natural choice for ordinary speech.

Because 聞くの is the subject of 楽しみです, so it takes が.

The sentence is structured like:

• [listening to my grandfather's stories] = subject
• is enjoyable / is something I look forward to

So:

• 聞くのが楽しみです = Listening ... is something I look forward to

Using を would be wrong here because 楽しみです is not a verb taking a direct object in this structure.

A useful way to think about it:

• X が 楽しみです = I look forward to X

Examples:

• 旅行が楽しみです = I’m looking forward to the trip.
• 会うのが楽しみです = I’m looking forward to meeting you.

This is an important difference.

• 楽しいです = is fun / enjoyable
• 楽しみです = is something I look forward to

So:

• 祖父の話を聞くのは楽しいです would mean Listening to my grandfather's stories is fun.
• 祖父の話を聞くのが楽しみです means I look forward to listening to my grandfather's stories.

In your sentence, 楽しみです expresses anticipation, not just enjoyment in the moment.

Japanese often leaves out subjects when they are clear from context.

So even though the sentence does not say 私は, it is naturally understood as something like:

• I look forward to listening to my grandfather's stories at family gatherings.

Adding 私は is possible, but often unnecessary unless you want contrast or emphasis.

For example:

• 私は、家族の集まりでは、祖父の話を聞くのが楽しみです。

This is grammatical, but the original sounds more natural in many situations.

In this sentence, yes, it will normally be understood as my grandfather.

Japanese family terms like 祖父 often depend on context:

• 祖父 can mean my grandfather or someone's grandfather, depending on the situation.
• In a personal sentence like this, it is naturally interpreted as my grandfather.

Also, Japanese often does not use words like my when the relationship is obvious.

Japanese nouns usually do not mark singular vs. plural the way English does.

So 祖父の話 could mean:

• my grandfather's story
• my grandfather's stories
• what my grandfather says

In this sentence, English often translates it naturally as my grandfather's stories, because that fits the idea of hearing him speak at family gatherings.

Specifically the grandfather's stories.

The topic phrase 家族の集まりでは sets the situation:

• At family gatherings ...

Then the main point is:

• 祖父の話を聞くのが楽しみです
• listening to my grandfather's stories is what I look forward to

So the focus is not simply family gatherings are enjoyable, but rather:

• At family gatherings, the thing I especially look forward to is hearing my grandfather talk.

Yes, it is possible, but the nuance changes a little.

• 家族の集まりで = simply gives the location/setting: at a family gathering
• 家族の集まりでは = makes that setting the topic and can add a slight contrastive feeling

So:

• 家族の集まりで、祖父の話を聞くのが楽しみです。
  = At family gatherings, I look forward to hearing my grandfather's stories.

• 家族の集まりでは、祖父の話を聞くのが楽しみです。
  = As for family gatherings, hearing my grandfather's stories is what I look forward to.

Both are natural, but では gives a bit more framing emphasis.