Hi Chang, and welcome to MySwissAlps!
>> If I have already bought a Swiss Pass and wanted to travel on Golden Pass Line from Luzern to Interlaken, can I use my Swiss Pass?
Yes, the Swiss Travel Pass (not Swiss Pass - that's something different, and not for tourists) provides free travel on the Golden Pass route.
You can download a map from the link below, showing what is covered by the Swiss Travel Pass.
As explained in the legend on the map, red lines are trains, yellow lines are buses, black lines are cable cars, white lines are boats. Solid lines are free, dotted lines are discounted. Discounts are mostly 50%, but only 25% for transport where you see the number "1" inside circle near the dotted line. There are a few grey lines on the map as well, which indicate transport that is not covered by the Swiss Travel Pass. Dashed red lines on the map are railway tunnels, and are on routes that are free with the Swiss Travel Pass.
>> do I still have to make a reservation on Golden Pass Line?
No, reservations are not required.
>> If a reservation is compulsory, then after making a reservation online, their reply is good enough to be used as a prove or they will just send me the reservation number?
For trains on which a reservation is compulsory (the Glacier Express, the Bernina Express and the Gotthard Panorama Express only) they send you an email with a PDF attached for printing out. The PDF has details of the route, departure time, class of travel, carriage number, seat number, name etc. An email is not good enough on its own.