Transaction Id (or Transaction Hash) is a unique id of every transaction received by Ethereum ETC blockchain. If usually takes few seconds for a transaction to be accepted by the network and displayed on Gastracker. If you have a Transaction Id that cannot be found on Gastracker is either means that transaction was not send to ETC blockchain or belongs to a different blockchain, such as ETH or testnet.
If you have a Transaction Id provided by an exchange (or third party wallet), received on withdrawal of your funds, and you don't see it on Gastracker for more that 15 minutes after making a withdrawal, it could mean that there were a connection problem on exchange side and the transaction wasn't transmitted to main ETC network. You need to contact exchange support to resolve the issue.