Get custom post fields and display them

get_post_custom_values() is meant for retrieving fields that have multiple entries.

Description

This function is useful if you wish to access a custom field that
is not unique, i.e. has more than 1 value associated with it.
Otherwise, you might wish to look at get_post_meta().

Returns an array containing all
the values of the custom fields with a particular key ($key)
of a post with ID $post_id (defaults to the current post if
unspecified).

Using it the way you are…

… isn’t going to work because you will get an array. That is in addition to the PHP syntax issue that you are not actually echoing anything.

You need to be using get_post_meta() instead, and echo the result:

while ( have_posts() ) {
  the_post(); ?>
  <span class="main"><?php echo get_post_meta("Gear",get_the_ID(),true); ?></span>
  <span class="main"><?php echo get_post_meta("Size",get_the_ID(),true); ?></span>
}

Honestly, while not strictly WordPress, you are probably going to be better off with slightly more complicated code:

$span = '<span class="main">$s</span>';
while ( have_posts() ) {
  the_post(); 
  $gear = get_post_meta("Gear",get_the_ID(),true);
  if (!empty($gear)) {
    sprintf($span,$gear);
  }
  $size = get_post_meta("Size",get_the_ID(),true);
  if (!empty($size)) {
    sprintf($span,$size);
  }
}