Boarding ends at 17:50 (per Stena Line website). You'll need to take an earlier train as the train station and port are a few km apart

You can take an earlier train from Oslo and enjoy Gothenburg for a while

Check-in in Gothenburg opens 45 minutes before departure and closes 30 minutes before departure:

https://www.stenalinetravel.com/customer-service/at-the-port/how-long-before-departure-do-i-need-to-check-in

The Stenaline terminal in Gothenburg for the ferry from Denmark (Masthugget terminal) seems to be around 2.2 km from Göteborg Central station. Tram lines 3, 9 and 11 pass close to the terminal.

That means that 40 minutes between (planned) arrival at the station and ferry departure is too short.

Thank you both so much for your informative replies. We will be travelling from Andalasnes via Oslo so unfortunately we cannot reach Gothenburg any earlier. We will stay in Gothenburg and catch a ferry the next morning now. Thank you 

Hi @Shaz 

I think that’s a good move. They don’t start boarding until just 30 minutes before departure, but there aren’t loads of foot passengers, so the short turnaround is completed quite quickly.

You may well know, but just in case and assuming your Interrail Pass is active, using discount code EURAIL2023 on the Stena Line website gets you a discount (up to 30%).

If you use Google Maps to find the ferry port in Gothenburg, this bridge (Stenabron) will take you right into the terminal and the machines that check you in / print the boarding pass.

We stayed in STF Stigbergsliden Hostel Göteborg, which is a ten-minute walk up the road from the ferry terminal. Shared toilet/bathroom but it’s nice and clean and a good budget option close to the ferries, with some nice places to eat/drink also easy to walk to (we liked Kulturhuset Oceanen and Blackbird).

@rvdborgt is spot on about getting the those trams. We bought our tram tickets in the Pressbyrån convenience store in Gothenburg Central Station. I read you can also get them on the Västtrafik app.

If you end up on Stena Danica, check out the Reef Bar on the top deck for good views out of big windows as the ferry leaves.

Just to be clear, Stena Line have 2 different routes from Gothenburg which depart from different places, make sure you go to the correct one for their Frederikshavn service as shown above.

Thank you both for your really good tips. I will book that hostel in a moment. Sorry for late reply .. got in Fauske late last night and then couldn’t find the accommodation in the dark . Off to Trondein shortly but wish we had another night here , it’s lovely. 
thank you so much again