I went to Skellige as part of Following the Thread and have not started Old Pals yet (my second playthrough) and was wondering if I'll be able to play Lambert at the Nowhere Inn so long as I don't progress his quest any further.
At any point you physically encounter Lambert prior to him reaching the meeting point outside of someone's house at the end of the Following the Thread quest you can interact with him again and initiate the Gwent game.
Additionally after completing his quest I visited the Nowhere Inn later and saw him sitting there rolling dice and was able to interact with him.
You should be able to play him after completing The Final Trial per usgamer.
This is the part of the quest that will require some patience, as you can only play Lambert after you travel to Kaer Morhen to complete the Ugly Baby main quest. At first you will not have the option to challenge him to a game of Gwent, but after completing The Final Trial the conversation option should appear. If you’re at Kaer Morhen and are having trouble finding him, make sure that you’re tracking the Old Pals quest and he should appear as an objective marker on your map.
You should be able to play him anytime you can speak to him, provided it does not automatically trigger quest continuation. I was able to play him before he went to Kaer Morhen.
I managed to play him in Novigrad after we visited the other witcher in his house.