You could try it with a callback function:

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading || newValue == '') {
        return;
    }

    checkNameSpace(callbackNameSpace);
}
function callbackNameSpace(boolOutcome) {
    if (boolOutcome) {
        //
    }
    else {

    }
}

function onLoad() {
}

checkNameSpace = function (callback) {
    var strField = 'tenant_namespace';
    g_form.hideFieldMsg(strField, true);
    var strName = g_form.getValue(strField) + '-' + g_form.getValue('tenant_environment') + '';
    if (strName.length < 6) {
        return true;
    }

    var ga = new GlideAjax('ValidationsAjax');
    ga.addParam('sysparm_name', 'checkNameSpace');
    ga.addParam('sysparm_environment', g_form.getValue('tenant_environment') + '');
    ga.addParam('sysparm_namespace', g_form.getValue(strField) + '');
    ga.getXMLAnswer(function (response) {
        processNS(response, strField, callback)
    });


    function processNS(response, strField, callback) {
        try {
            var answer = response;
            if (typeof callback === "function") {
                callback(answer);
            }

        } catch (e) {
            console.log('check ns error : ' + e);
        }
    }

};