It's hard to tell from your code sample what you're up to.
What you're searching for is the enum esriFieldType which can be tested against each field to determine its type:
MyComboBox.items.clear(); // empty first
IFeatureLayer pFtLayer = (IFeatureLayer)gMap.get_Layer(0);
IFeatureClass pFtClass = pFtLayer.FeatureClass;
for (int Cnt = 0; Cnt < pFtClass.Fields.FieldCount; Cnt++)
if (pFtClass.Fields.get_Field(Cnt).Type == esriFieldType.esriFieldTypeString)
Assuming your feature class comes from the first layer and your combobox is called MyComboBox... beware to set your Combobox.DropDownStyle to ComboBoxStyle.DropDownList to avoid users (accidentally) typing values in, this would mean that the value in MyComboBox.Text must be one of the items in the list or blank.