I have been a personal trainer at a gym for 10 years.
I have been a personal trainer at a gym for the past 10 years.
I have been a personal trainer for 10 years.
一つ目の回答は、以下のような日本語を意味します。
ジムで10年間パーソナルトレーナーをしていました。
「I have been a personal trainer at a gym for 10 years.」
二つ目の回答も同じ意味ですが、直近の10年間ジムで働いていた場合に使えます。
「I have been a personal trainer at a gym for the past 10 years.」(assuming they are still a personal trainer at the gym and that it was the most recent 10 years)
三つ目の回答では、ジムという単語を使っていません。
普通はジムのパーソナルトレーナーだと受け取るので、特に言わなくてもOKです。
「I have been a personal trainer for 10 years.」(You don't really need to specify whether you were a trainer at a gym, as many people would assume that a PT works at a gym anyway, but if you want to specify that it is at a gym and not freelance then you could also say where.)
I hope this will help you.
お役に立てば嬉しいです