Most of Longview is in Gregg County. The link to the Gregg County Central Appraisal District is: http://www.gcad.org/ This site should have last years tax info for this property.
A part of Longview is in Harrison County. If you did not find your property on the above site, check here:
http://isouthwestdata.com/corp/ Pick "Harrison" in the Special Appraisal District drop down.
Hope this helps!