1- My job offer is at Tenneco automotive, which is 5km from Rynek (a few hundred meters east of the Aeroklub Gliwice Trynek), do you know if there is any public transportation to the place (Tenneco)?
There is. It seems the closest bus stop to your future (?) workplace would be 'Gliwice Fadom' (lines 8 and 58).
Example: from 'Plac Piastów' (close to Gliwice's railway station) to 'Gliwice Fadom' => 12 minutes (line 58).
Look here.
rozklady.kzkgop.pl/kzkgo/index.php?co=mapa
(and choose 'english')
I worried about riding there by bike during winter.
If it's a snowy winter, that would be complicated I think.
2 - Is it better to rent an apartment once I'm at the city or before arrival?
You'd better start looking for offers before you go and maybe get some appointments.
When I went there, I found my apartment very quickly (a couple of days). A friend of mine lived there for a while too and she didn't speak a word of Polish. She didn't have much trouble finding a nice apartment either.