Does 200 transactions limit apply to gems?

Nope, the 200 limit goes for all items, even cards/emoticons.

Once you hit 200 transactions for the first time, you just have to fill a form and no more limits, so you shouldn't really care.

Yea, I don't have any limits. I buy and sell hundreds of items at a time on the market.

Just fill uneasy to fill some forms with personal info.

I wouldn't worry about filling out the form, it's no big deal unless you plan on making more thank 20k in a year, then they'll report that gross income to the IRS.