Hi. I'll get straight to it.

1: Been trying to understand/win at gwent, but i could only beat the tutorial professor guy. Is there any in depth guides out there? Really want to get better at Gwent. (Currently i am trying to beat the nobleman after the Imperial Audience)

2: Should i go to Velen before Novigrad and Skellige? Yennefer said i should, but i'm not sure. I want to meet Triss, who is in Novigrad. Or does going where i want ruin the quest line?

Here you go, this seems about right. This is how quest log looks like in Witcher 3. See the "Suggested level" thingy? All of the main quests have those and it tells you at which level should you attempt that particular quest. As you can see, you should go into Novigrad once you reach level 10, and Skellige at level 16.

Regarding the first - you might want to do a google search for this if you want an in-depth guide. What I can tell you right now - your beginners deck is really not up to much, so you might really just try to play a lot of innkeepers or traders etc, so you can win better cards, also (obviously) buy all the cards you possibly can. The nobleman in Wyzima can be a bit tricky, he does have some nice hero cards IIRC, so you might want to let this one sit for a bit.
When assembling a deck go for eiither Northern Kingdoms or Nilfgaard (other two decks are way weaker, and you only get good cards a lot later in the game).
For general strategy - have some effects card (weather, decoy etc, I find a max of 6/7 quite practical), apart from that, keep your deck neat and tidy and replace weaker cards as soon as you can. What has almost always worked for me - toss the first round, try to weaken your opponent's deck as much as possible by making him overcommit, second round is usually an easy win after that, third round is where it's at ;)

As for the second, as fenixp stated, the lvls are a good hint. You can, however, mix it up (I always went to Skellige for some minor quests fairly early on), those are just recommendations after all. You will not mess up any quests if you don't progress too far in the main quest line. Going to Novigrad fairly early on is fine (for example to do some shopping, meet Triss or whatever), but for a more coherent story you should probably pursue the main quest in Velen for a bit, things might get a bit confusing otherwise.

Thanks guys, i understand Gwent a little better now. However, i have come across a more annoying issue right now.

Why am i underleveled? I just entered Velen and i am level 4. All quests seem above me. Did i miss something in White Orchard? I did every question mark there.

I guess i am forced to do main quest? Won't the main quest ruin some side quests?

I think lvl 4 is about average after White Orchard (and yes, you did well to do the question marks there, so you should have some money/ability points?).
The level system in TW3 is a bit flawed, it is important to remember that the level recommendations are only recommendations. You will be fine doing green quests (your lvl +5), and sometimes "red" quests, especially on easier difficulties and once you get the hang of combat.
And yes, following the main quest for a bit is probably the best course of action - also, visit as many notice boards as possible for additional quests, contracts and points of interest, that way you'll find more quests within your "range"... (also, try to do all quests while they are still green for max XP gain, otherwise they only grant a negligible amount of xp).
The main quest will not ruin sidequests for you (there's one cutoff point about two-thirds through the game, but the game does warn you that some quests might auto-fail if you proceed). There are very few sidequests that are missable at all, and it's always quite obvious which ones that may refer to. So, no need to worry :)

Thank you! Does completing that quest line end any sidequests? I've heard doing main quests can ruin side quests

No, it does not ruin any side quests. You may get side quests during main story quests, but the only point that the main story puts a stop to anything is at a pivot point in the game, where you are warned that going on is a 'no turning back' stage. The only way to ruin side quests, if you could call it that, is to do them when your character is more than 5 levels above the recommended, since you will get little to no XP.