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Edit  dc
while doing something and taking advantage of that opportunity to do something else
getting one thing done while doing something else
Edit  dc
See also
Since I took a walk. I stopped at a bookshop.
Edit  #3278 Miki
Since I've come all the way to the capital, I might as well get some shopping done on top of sightseeing.
Edit  #6196 colinp123
Since you go for a run in the car, coud you pick me up at the station?
Edit  #4741 miki
Chat with a colleague on the internet for a change.
Edit  #4742 miki
Discussion and comments
Used to express getting one thing accomplished while doing a second thing. Perhaps a good English equivalent would be "to kill two birds with one stone."
more formal than -nagara but similar - ??
gatera can be reaplced with [tsuideni]ついでに or wo[kanete] を兼ねて.
x散歩をしながら、本屋に寄った。This is not natural.
I think that がてら is used to show another action (本屋に寄る) taken place in the middle of one main action (散歩をする), 散歩→本屋→散歩
While ながら、two or more actions are at the same time.
eg. 友だちと喋りながら歩く。 I walk as talking with a friend. 歩く=喋る
You can also use かたがた.
got it. so its not a verb ending like 〜ながら。
It means more like "on the occasion of" or "while doing A, also did B" than "during or while" ... ie slightly separated incidents.

Is it a very formal/written phrase?
Right on, dc.

ついでに is conversational, がてら,かねて and かたがた are predominantly used in writing.
My book says that this is not used for a "big" event. It is used for something small or something that can be finished soon.
旅行がてら = NO
散歩がてら = OK
No, that is not correct.がてら can be used in any situation where you say "while doing X, I also did Y."

There is such limitation as Blabby refers to.

It is perfectly correct to say 中国旅行がてらタイにも足を伸ばした。Or 花子と話がてら、夏子にも挨拶した。

I dn't know what your book is, but if that's what it says,it is not correct.
The book I am using is 実力アップ!日本語能力試験一級文法編 by Unicom. The explanations are in Japanese so Ill quote from the book.

〜がてら ー (〜をするときに)
×旅行がてら  ○散歩がてる

Maybe this books explanations are not as good as I thought...
I think that "gatera" is most like "tsuideni". It means "on the occasion of".
To Mikisan: Vる/N の+ ついでに
--> 散歩のついでに....
がてら has two meanings.
1.verb + がてら + A
In this case, がてら is used to show another action(A) taken place in the middle of one main action. This can be replaced with ついでに, but not with 兼ねて.

2. noun(A) + がてら + B
In this case, it can be replaced with ついでに or 兼ねて. But be carefull, there's a subtle difference between the two.
ついでに = doing B while doing A
兼ねて = doing B with the intention of doing A
In the book くらべてわかる中級日本語阜サ文型ドリル, they talk about the fact that it depends on the location where the 2 actions take place.
The 2 actions take place in same or different location -> AついでにB
The 2 actions take place in different location -> AがてらB(unless the main action implies a movement or displacement 〜しに行く)
Also the 2nd meaning of AがてらB could be "doing B will help to do A" and also only ついでに can be used at the beginning of a sentence.
But the two expressions can mean "killing 2 birds with one stone".

In fact, after reading many exemple sentences, I think that the difference between がてら and ついでに
doesn't lie in the meaning (beside the 2nd meaning of がてら).
Most of the time they mean the same. What makes me choose one over the other is how I construct the sentence, which words I use and how the whole sentence sounds.
In the end, I believe one shouldn't try too hard to build a rule to differentiate them.
If a rule is not obvious enough or contains too much exceptions, then it shouldn't be used as a rule in the first place.
I think it's one of those cases where you have 2 ways for saying the same thing, which is great so when you talk, you don't sound all the same.