The following code will add “USD” to the end of the U.S. Dollar currency in Gravity Forms.
For example, instead of seeing $34.20 – $34.20 USD is displayed.
This will not affect existing submissions – only newly loaded forms and new submissions.

If you’re not sure where to place this code I highly recommend you read How to create a WordPress plugin for your custom functions.
add_filter( 'gform_currencies', 'gg_gf_usd_suffix' );
function gg_gf_usd_suffix( $currencies ) {
$currencies['USD'] = array(
'name' => __( 'U.S. Dollar', 'gravityforms' ),
'symbol_left' => '$',
'symbol_right' => ' USD',
'symbol_padding' => '',
'thousand_separator' => ',',
'decimal_separator' => '.',
'decimals' => 2
);
return $currencies;
}
This uses the gform_currencies filter that allows you to update or create currencies. See the Gravity Forms documentation for examples of how to use this filter.