Hi, I analysed around 130 games and here is how the MMR in Gwent functions.

What I found:

Score, cards remaining, or the number of rounds won’t affect the MMR/points you gain or lose. The major factor that determines the MMR you gain or lose is the rank difference between you and your opponent. Your rank will also affect the MMR but only when you are losing (see explanation below).

• Winning

Winning is easy to calculate because your rank doesn’t matter. Generally, it works how we expected it to be. The higher the MMR you have than your opponent, the less MMR you gain when you win, vice versa (look at the graph to have a better idea how much). If you want to calculate how much points you will gain, use the formula in the graph below.

Graph

• Losing

Losing is much harder to calculate, because your rank matters. The higher the rank you have, the more point you will lose. I have two graphs here showing the difference. Even though these two graphs have a similar pattern, we can see that when facing the same MMR opponent, we lose around 33 points at rank 6, but 43 points at rank 14. Much more data at different rank will be needed to verify how it really works, it is too time consuming for me so I stopped after collecting around 130 games at these two different rank.

Graph for rank 6

Graph for rank 14

• Draw

Draw or disconnect that led to draw, depend on whether you have higher or lower MMR than your opponent. If you have higher MMR, you lose points. However, the points you lose will be much lower whem compared to winning/losing. Vice versa, you gain points when you have lower MMR.

Raw data

P.S. The sample size is only 130 games (which is small), so it is possible that I misanalysed and errors might exist. Leave a comment if you find any errors. Thank you for reading.

TL;DR
• One major factor that affect the MMR you gain/lose is the MMR difference between you and your opponent.

• Calculating the points when you win is easy because it is the same at all rank.

• Losing is different, you lose more point when you are at high rank, which mean climbing at lower rank is easier.

• Win rate to climb at around rank 6: ~40.5%

• Win rate to climb at around rank 14: ~46.7%