Depending how you have thinsg set up you could use the category_group parameter to include or exclude a whole group of categories.
This would list all entries not in category group ID2
{exp:channel:entries channel="mychannel" category_group="not 2"}
Uncategorised entries
{/exp:channel:entries}
This would list list all entries that have been assigned a category from group ID2
{exp:channel:entries channel="mychannel" category_group="2"}
Categorised entries
{/exp:channel:entries}
You could run a conditional in your listings template to test whether a category is being called, eg
{if segment_2 != "category"}
{exp:channel:entries channel="mychannel" category_group="not 2"}
Uncategorised entries
{/exp:channel:entries}
{/if}
{if segment_2 == "category"}
{exp:channel:entries channel="mychannel" category_group="2"}
Categorised entries
{/exp:channel:entries}
{/if}