I can explain.
The key point is: you have a Rank and an MMR that are not entirely linked together, and matchmaking only look at your MMR to decide who goes in your game. THEN, the system look at BOTH your Rank and your MMR to decide how many LP you win or lose.
The matchmaking goal is ALWAYS to give you fair matches. Always. It tries to make two teams that are as good as each other. (Sometimes it just fails, there are some errors, but then it's a mistake in the execution, not in the design. The design is that each team always has a fair chance to win, and that's a good design.) So if you are matched against a Gold player, that's a good sign, it means the match-making system thinks you are as good as that Gold player (or the average of that Gold player and their premade).
The problem with that approach is, if the system thinks you are as good as a high-Silver/Gold, but you are still in Bronze, it's frustrating. You know you deserve Silver or even Gold, but you don't have that Rank displayed on your profile, and it's unfair. True. And that's why there is the second part, the part about how many LP you win or lose. If the system thinks you're high-Silver but you're officially Bronze 1, you will earn a shit-ton of LP for each Victory, and lose very few LP for each Defeat. Typically, +26-30 LP / -15-17 LP. So even with a 50% winrate, you will get (+28) + (-16) = +12 LP for 2 games. And you will climb, and you will get out of Bronze eventually. Even by being matched against Gold.
And what about Promo Series? Well, that's why you have free wins. For same-tiers series (Bronze3 to Bronze2), you have 1 free win for a Bo3, so you need to win 1 of 2 games, that's still 50%. For a series to the next tiers (Bronze1 to Silver4), you can have up to 2 free win for a Bo5, so you need to win 1 of 3 games, that's 33%, so it's even easier.
tl;dr: being matched against Gold is a good sign, and the system is fair. Just keep playing and you will climb.