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The grammar dictionary
[どんなときどう使う日本語表現文型辞典](https://books.google.co.jp/books/about/%E6%96%B0%E8%A3%85%E7%89%88%E3%81%A9%E3%82%93%E3%81%AA%E3%81%A8%E3%81%8D%E3%81%A9%E3%81%86%E4%BD%BF%E3%81%86%E6%97%A5.html?id=Kq0Yt_ZbligC)
defines がてら in the following way:

>   1. Accomplishing two objectives via one action. Can also mean to produce
> two results by doing a single action.
>   2. Verbs of movement, such as 歩くor 行く, often follow.
>

It also gives the following example:

> ② 散歩がてら、ちょっと郵便局まで行ってきます。

The sentence ② is basically consistent with the example given in [this
question](https://japanese.stackexchange.com/questions/28667/difference-
between-%E3%81%8B%E3%81%9F%E3%81%8C%E3%81%9F-and-%E3%81%8C%E3%81%A6%E3%82%89),
and the explanation given in [the
answer](https://japanese.stackexchange.com/a/28670/11104): " _the phrase
散歩がてら立ち寄った公園 would make more sense because stopping at a park is done while on
the walk._ "

It is also consistent with the explanation given in [this
answer](https://japanese.stackexchange.com/a/5928/11104): " _運転(し)がてら食事する --
eating along the way driving (after having driven half way)_ "

* * *

Now, the problem is that the same dictionary gives also another example:

> ④ 駅まで３０分ほどかかるが、天気のいい日は 運動がてら歩くこと にしている。

and I see it as contrary to the above.

Walking is neither done in the middle of an exercise; nor is exercising the
main action here (as we are talking about the station, then the action of
moving towards the station should be, no matter what).

* * *

Why isn't the example written in the following way:

> 駅まで３０分ほどかかるが、天気のいい日は 歩くことがてら運動 にしている。

Would there be a difference in meaning? What would be the meaning then?

Even if, for some reason, it doesn't sound right for a native speaker, please
explain what meaning it would convey.

Does it matter that the predicate is making decision (にする) here?
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question
grammar
What is the main action with がてら (in the given example)?
"運動がてら歩くことにしている" is OK but I prefer 歩きがてら運動することにしている.

歩くことがてら is simply ungrammatical and 運動することにしている means "I am scheduled (at my
own will) to exercise myself" while 運動にしている means "I make it into exercise".
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> 散歩 **がてら** 、ちょっと郵便局まで行ってきます。
>
> 駅まで３０分ほどかかるが、天気のいい日は 運動 **がてら** 歩くこと にしている。

It's a kind of long walk that it takes 30 min to the station, but it makes a
good exercise, so the speaker chooses to walk when the weather is good. The
speaker is talking about **_walking_** to the station. The がてら phrase is an
adverbial phrase; the sentence stands without it.

If it's 駅まで３０分ほどかかるが、天気のいい日は歩きがてら運動することにしている, it's talking about the
_exercise_ done by the long walk. Here I find the adverbial phrase is
something more important than a mere modifier; you can't omit 歩きがてら.
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According to 明鏡国語辞典, 「～がてら」 can mean either 「～のついでに」 or 「～を兼ねて」.

And it gives two examples:

> A: 「駅に行きがてら郵便局に立ち寄る」  
>  B: 「散歩がてら買い物に行く」

The がてら in A means のついでに. I think the one in B can be interpreted as either
のついでに or を兼ねて.

A: "I drop by at the post office along the way to the station."  
B: "I go shopping while taking a walk." OR "I go shopping and it also/partly
serves as a walk."

So I think your sentences:

> ② 散歩がてら、ちょっと郵便局まで行ってきます。  
>  ④ 駅まで３０分ほどかかるが、天気のいい日は運動がてら歩くことにしている。

The がてら in ② can be either 「を兼ねて」 "partly for the purpose of~~" "which also
serves as~~" or 「のついでに」 "while doing~~" "along the way". The one in ④ is used
in the sense of 「を兼ねて」.

> 「駅まで３０分ほどかかるが、天気のいい日は（駅まで）歩きがてら運動することにしている」

I think it would mean "I (regularly/usually) get some exercise while walking
to the station." (but it doesn't sound very natural to me...)
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