Looking at this purely from the WMS interface standards perspective, we can see that there is no default format for a GetFeatureInfo request (so if OpenLayers suggests it is GML, then that is a mistake).
Looking at WMS 1.1.1 we can see that the info_format parameter is optional (so any software compliant to this specification can define its own default). As far as I can tell both GeoServer and MapServer default to plain text as the default when making a WMS 1.1.1 GetFeatureInfo request and not specifying the info_format parameter, or specifying the parameter as empty like
For the WMS 1.3.0 specification there is still no default format specified, but this time there is a requirement to supply an info_format parameter as part of your GetFeatureInfo request.
It might be that OpenLayers defaults to using GML as the info_format for a GetFeatureInfo request, and if so it can only guarantee such a format by requesting it using the info_format parameter.
To ensure an HTML response you would be wise to always specify that format using the
INFO_FORMAT=text/html& as part of your GetFeatureInfo request.