Make sure you iterate through your gridview rows after you bind it to data.
protected void Page_Load(object sender, EventArgs e)
{
if(!Page.IsPostBack)
{
gvExcParts.DataSource = yourDataSource;
gvExcParts.DataBind();
for (int i = 0; i < gvExcParts.Rows.Count; i++)
{
TextBox text = (TextBox)gvExcParts.Rows[i].FindControl("txtRMAPartQuantity");
}
}
}