I will be flying from Dusseldorf to Stockholm on Eurowings and have been trying to find out if access to the lounge is granted being a *Gold.
You are s Star Gold member with which program?
I didn't think there was but it seems that some UA *Gold members were allowed to use the LH business class lounge.
All UA Gold members should get access to Lufthansa Senator and Lufthansa Business Lounge, when departing on a Eurowings-operated flight, regardless of fare class or fare family. However, it does only apply to Lufthansa lounges, not other airline lounges (e.g. Austrian in VIE, Swiss in ZRH).
http://www.lufthansa.com/uk/en/Lounge-types-and-access
Passengers who have a Star Alliance Gold Card, however, do not have access to the Lufthansa Lounges. Exceptions to this are the Star Alliance Gold Card customers of the airlines United Airlines (UA) and All Nippon Airways (NH), who receive access to Lufthansa’s Senator and Business Lounges.
I recommend to print out this page. LHR LH Lounge is well aware of this rule. However, smaller LH outstations with own lounges (e.g. NUR, STR, TXL) may not know this.
What is not clear is the companion rule.
(6) To access Senator and Business Class Lounges travelling companions must present a valid boarding pass for a same-day departure Lufthansa / Star Alliance flight.
Literally speaking if the UA gold card holder departs on Eurowings, but the companion on (lets say) LX, OS, LH, SN -> then access is granted.
If the companion travels on the same flight of the companion, then access is denied to the companion.