I have two columns grouped under a single header. Resizing any column causes the headers to become mismatched from the rest of the grid.  NOTE: This error occurs in your demo as well…

https://www.gridphp.com/demo/demos/appearance/group-headers.php

Column definitions…

$col = array();
$col["title"] = "Contract"; // caption of column
$col["name"] = "msContrHeads"; // grid column name, must be exactly same as returned column-name from sql (tablefield or field-alias)
$col["width"] = "60";
$col["hidden"] = false;
$col["search"] = false; // this column is not searchable
$col["editable"] = true; // this column is editable
$col["editrules"] = array("number"=>true);
$colsMS[] = $col;

$col = array();
$col["title"] = "Current"; // caption of column
$col["name"] = "msCurrHeads"; // grid column name, must be exactly same as returned column-name from sql (tablefield or field-alias)
$col["width"] = "60";
$col["hidden"] = false;
$col["search"] = false; // this column is not searchable
$col["editable"] = true; // this column is editable
$col["editrules"] = array("number"=>true);
$colsMS[] = $col;

SETTING THE GROUP HEADERS...

$g2->set_group_header( array(
        "useColSpanStyle"=>true,    // NOTE: problem occurs whether this is true or false
        "groupHeaders"=>array(
            array(
                "startColumnName"=>'msContrHeads', // group starts from this column
                "numberOfColumns"=>2, // group span to next 2 columns
                "titleText"=>'Head Counts' // caption of group header
            )
        )
    )
);