That person's suggestion is correct--in a sense.
"Kinka wo atsumete imasu" is the closest translation of "I collect gold coins."
"Kinka" means gold coinage, too.
However, Japanese and English are different languages and there are no 100% equivalents (most of the time) because of the difference in approach that stems from culture and history.
"Kinka wo atsumete imasu" can mean "I collect gold coins (as a hobby)" AND "I am collecting coins (now--meaning you are physically gathering coins that may be scattered around where you're standing)." The interpretation depends on context of when and in what situation it is spoken.
"Kinka" can also be interpreted as "gold coins (as individual pieces)" and "coinage/currency in gold," also depending on context of usage.
In short, he is right, depending on situation.
Your signature "watashi no shumi wa kinka atsume desu" literally means "my hobby is collecting coins." You may not feel this is not exactly what you want, but this expression minimizes the chance of misinterpretation. So, I think your signature is OK.
As you advance in your Japanese language skill, you'll find that you should not expect "100% exact" translation of many sentences between English and Japanese.
Hope it helps!