Since Gearbox Protocol has two sides to it, the fees you are paying depend on what user type you are. If you are a passive Liquidity Provider, there is no fee except the regular lending protocols' APY spread going into protocol treasury + a withdrawal fee in the first weeks of the protocol launch. See more here. If you are a leverage user, you are paying a borrow interest fee (which fluctuates depending on utilization ratio) + liquidation fees to the protocol + liquidator if you are liquidated.
Diesel Tokens are like cTokens of Compound. These tokens automatically earn interest & fees proportional to your share of the pool like cTokens on Compound or Yearn LP tokens. You don’t need to claim interest or perform any other actions, your Diesel Tokens value is supposed to grow.
Health Factor is a numeric representation of your account health. If your health factor drops below 1 or close to it, you might be liquidated. The higher the number is, the safer you are. Since liquidations make you incur higher fees, make sure to follow tips on how to not get liquidated.
Depending on the utilization ratio of pools, if it is high - not all the liquidity can be withdrawn at once. It doesn't mean that the protocol lost money or is insolvent - it's just that the liquidity is utilized in Credit Accounts. In such cases where available liquidity is low (which you can check in the app analytics page per pool) - the APY goes higher as to attract more capital providers and attempt to find an equilibrium. This is more or less standard among all lending-type protocols.
Why can't I borrow more?
As the protocol has max leverage and max personal borrow parameters, which are configurable by DAO, there is a limited amount post-multiplier you can borrow for leverage. As such, borrowing above that amount is technically not possible. Once a Credit Account is open, you can add more collateral to increase your position size and Health Factor, but you won't be able to borrow more than the notional starting number - basically, there is max debt on borrowing.
Why is mobile not accessible yet?
As the protocol has gone live just recently, and there was no previous testing on mainnet (only on Kovan) - nobody knows what PMF (product-market fit) Gearbox can have. The app will likely take many iterations, and anybody can build their own. So while this search for users and market fit are ongoing, the effort on optimiizing 100 versions at once is minimized. Later on, as things become more clear - the mobile version will be optimized and turned on.
Will Gearbox Protocol be on other chains/L2/rollups/EVM-compatible/etc. keywords?
Gearbox Protocol is DAO-operated, so it should be a joint community decision. That's one. If users need Gearbox, it's fine to deploy anywhere, because user-first approach.
But it's not practical being on chains where there are not many other protocols to tap into. You see, Gearbox sources liquidity from other DeFi protocols: trading on Uniswap and Curve, farming in Yearn & etc., and more. If a chain only has a Uniswap (original or fork) and a Curve (original or fork) it is  less safe usually  what is more important - it makes the product of Gearbox be inferior. If there is not much liquidity or you can't build composable positions, Gearbox becomes a short-long interface basic thing only which is like 10% of its value proposition.
So in the future there is no worry with it, but current ecosystems need to develop more and have more safe and liquid protocols to justify time and development costs. Which is likely possible, and is something to altogether discuss in 2022.
Why can't I trade ETH on DEXes via Wallet Connect?
Because ETH is not an ERC20 token by itself, within the Credit Account ETH borrowed is converted to WETH. So if you connect via Wallet Connect or see your Credit Account on Etherscan - and check for ETH, you will see WETH. After liquidation or closing your account, it is converted back to ETH, so liquidity providers don't need to think through this too much.
Why does a trade from stable to stable decrease my HF (health factor)?
Due to how Threshold Weighted Values are calculated taking into account a possible price drop and a price feed tick, a stablecoin is not actually a stablecoin for the protocol contracts. Stablecoins tend to de-peg, which must be taken into account. This keeps the protocol more secure, but also causes max leverage parameter to be lower (as not the entire possible amount is extracted). As such, this is up to the risk appetite of Gearbox DAO to decide on such things.