Rabby absolutly respect and use tx params set by DApp, but somtimes DApp will not set some params, this doc will told you when and how Rabby change these params.
gas means gas limit for this transaction, if DApp not set it, we will set it an empty string ('') for explain (not execute actually, just for explain how this tx execute), after explain, API will told us gas limit of this transaction (1.5 * result of web3.eth_estimateGas() currently).
gasPrice means how much you will pay for each step of this transaction, if DApp not set it, we will ask API for GasMarket(which will response a list of gas and estimate time) first, then use the fastest one as default, user can change it in confirm page.
Sometimes DApp not set value param(like https://pancakeswap.finance/farms, when you enable a Stake LP, value is not set), we will set 0x0 as default.
When transaction is not a contract(like send token to another address), DApp will not set data param, we use 0x as default.