Wallet gas: the small balance every wallet needs
Wallet gas is simply the native coin sitting in your wallet to pay network fees. Without it, your wallet can receive funds but cannot send them.
Receiving works, sending does not
Anyone can send tokens to your address without your wallet paying anything. The fee is only charged to the wallet that signs an outgoing transaction.
That is why tokens can appear in a wallet that then refuses to move them.
How much to keep
Keep enough for several transactions rather than exactly one, because fees move with network demand and contract transfers cost more than simple sends.
On TRON, token transfers are noticeably heavier than plain TRX sends, so keep a wider margin there.
One balance per network
Balances do not travel between networks. ETH on Ethereum cannot pay Base fees, and BNB cannot pay Polygon fees.
If you use several networks, each one needs its own small gas balance.
Common questions
How much gas should I keep in my wallet?
Enough for several ordinary transactions on that network. Cheap networks need very little; Ethereum mainnet needs more at busy times.
Does the same gas work on every network?
No. Each network needs its own native coin held on that network.