Gdańsk is a city in Poland, along the Northeast coastline. Together with Sopot and Gdynia it makes large conurbation called Trójmiasto (Tricity), which is inhabited by nearly 750 thousand people. Tricity is connected by ferries with Sweden. Cars registered in Gdańsk have registration plates GD.
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- By expressway bypass S6 (E28), from petrol station
From the centre of Gdańsk take any local train called SKM going north and get off at Gdańsk Żabianka. Than take free bus 807 going to Osowa Shopping Centre which is located next to S6 (E28) Tricity Bypass. From the shopping centre go north along the bypass (follow the path on the grass field), get on the junction bridge and walk to the other side of the bypass. You will see petrol stations on both sides of the S6, north from the junction. Go around Castorama shop to get to "your" station. See satellite image of the spot in Hitchwiki maps.
Went there and actually bus 807 doesn't exist. If you look on the link there are no traces of this bus on the city's public transport website. If you want to go to Osowa, you need to take a special train that goes from Wrzeszcz station. Click on the link above to find the best connection from the point you are leaving. Didn't try it myself but might be a good option to leave the town.
The idea is good just bus 807 doesn't exist for a long time. For today (2018) : take SKM train and get off at Gdańsk Oliwa station. Than go to the tram/bus stop located 5 minutes from station :(https://www.google.pl/maps/place/Pętla+tramwajowa+Oliwafirstname.lastname@example.org,18.5670324,16.25z/data=!4m5!3m4!1s0x46fd753328412381:0x4b1f507901e623a4!8m2!3d54.4098998!4d18.5666002). Take bus 171/169 (timetables : https://ztm.trojmiasto.pl) and get off on Osowa-Obwodnica stop. 200 m. from it, there's junction and both petrol station are visible.
- By national road 1 (E75) or motorway A1, from bus stop close to old town
From main railway station in Gdańsk go south along Podwale Grodzkie and Wały Jagielońskie streets, cross roundabout and continue walking south along Okopowa street, cross rail tracks by the bridge and you can hitch on the first bus stop behind it. Here' the route
Another and in my opinion better option : take tram nr. 2/4/6/7 (destinantion : Łostowice-Świętokrzyska) from city center. Get off on Wilanowska stop (~10 min. journey from old town). Walk 100 m. north from tram stop and cross big street (Armii Krajowej). At the exit of avenue usunused bus stop is located. 
- By national road 1 (E75)
Another option is to use the local train called SKM, or regular train and get to Pszczółki or some other village between Gdańsk and Tczew. When you get off the train just walk out of the village in the direction of the road, it's visible from the station. Since first part of the motorway A1 was built there is less traffic on national road 1.
Take bus 112, 166, 186 or 212 to the stop called Miałki Szlak on Elbląska road, which is an excellent hitching spot. The bus stop after that (Węzeł Elbląska) seems like a more obvious choice at first, as it's located directly before the motorway turnoff, however there is a traffic light right before Miałki Szlak so people can check you out while waiting for the light to turn green - very helpful as cars go fairly fast on this busy road. Miałki Szlak might be an "on demand" stop so you need to press the "stop" button inside the bus.
- By expressway bypass S6 (E28) > national road 6, from petrol station
Get to Osowa Shopping Centre (see direction Toruń, Łódź by S6 for details). Once you're there go north along the bypass (follow the path on the grass field), go under the junction bridge and you will see Lotos petrol station where you can ask for a ride. It's also possible to hitch on the slip road next to the station.
- By national road 6 (E28), from bus stop behind the end of expressway bypass S6
This spot is located in Gdynia. Take any local train called SKM going north and get off at Gdynia Chylonia. Go south to the main road, turn right (west). Just behind roundabout (where S6 city bypass ends) is short lay-by. You can try hitching there, but some 200m further is bus stop with plenty of space to stop and just behind crossroad with traffic lights. Make a sign as there is a lot of local traffic.
- By national road 6 (E28), from bus stop in Wejherowo
This spot is located in Wejherowo. Take any local train called SKM going north and get off at Wejherowo (all SKM lines going north run at least to this stop), the ticket should cost around 3-4PLN or less with student card and the ride should take about 30 minutes. Once you arrive, walk down the log hall way of the station exit up the stairs and you should be standing next to the beginning of the national road 6 going in the direction of Koszalin, Szczecin (if you are in a small town, turn around and go back to the other side of the station). Once there, walk 30 meters back to the bus stop loads of space to pull over.
"If you go to Berlin it's better to write Polish city on the sign e.g. Szczecin (only 80km from Berlin) because even if Poles are going to Germany they may think you are German and may be shy to speak German (a Berlin sign took me much longer compared to a Szczecin sign for different trips)." - Anonymous
There is a good website showing you how to get from one point to another by public transport in major Polish cities (in Polish and English).
An excellent place to camp is the Oliwa forest which is along the road to Sopot. It is an enormous forested park, safe for putting up a tent. You can get there taking tram #12 from Gdańsk center.