WordPress function get_date_from_gmt($post->post_modified_gmt) usually returns a date and time in your site time zone. However, if this function is called from bbPress page, it always returns GMT time.
I started to dig dipper. Function depends on GMT offset returned by get_option(‘gmt_offset’). When called from bbPress this function returns nothing. As a result get_date_from_gmt() always returns GMT time.
Here is a chart:
function get_date_from_gmt() is in wp-includesformatting.php
function get_option() is in wp-includesfunctions.php