Using GF dropdown instead of currently form

Hi.
I´m currently using the form below with a script to push a selection to a php-file with shows the choosen sql-entries in a table.
How can i use a GF dropdown form instead (i´m a php-beginner and need it explained ;-)

<form>
<select name="users" onchange="showUser(this.value)">
<option value="">Model:</option>
<option value="1">Samsung Galaxy S 3</option>
<option value="2">Nokia Lumia 800</option>
<option value="3">HTC One X</option>
<option value="174">Apple iPhone 4 8GB</option>
</select>
</form>

I´m using a javascript to make a table from specific SQL-entries. I select which entries to show with a normal dropdown form (see above), but would like to use a Gravity Form instead (easier to edit and nicer looking etc.).
I think i have to do something with the
<select name="users" onchange="showUser(this.value)">
from the current form, but do not know where to put it on a GF dropdown form.