So, am I at fault here? Does Agent have any chance to vacant property legally in absence of latest GSC?
what should be done now? is it okay to arrange new before hearing? Also my EPC expire in July do I have to renew new ?
So you don't have the criminal legal obligation to keep the GSC up to date. That's on the Let-2-Let company who's the immediate landlord of the occupier. And from what you wrote, you don't have a contractual obligation to do so either as your obligation is to provide one or pay for it. And also like, you're not the occupier's landlord. If you were to attempt to arrange one, as far as the occupier concerned, you're a random stranger wanting access to their home for "checks", like "no? get out!".
If there's no up to date GSC, the L2L company is not going to be able to evict via section 21. Since there was one when the occupier moved in, they can get a new check done and re-serve the s21. Alternatively, the L2L can always try to incentivise them to leave by offering money etc. Having said that, if the L2L is like most out there, they're a small company with no realisable assets, push come to shove, the operators could just fold the comapny leaving you high and dry. One of the reason why L2L is not a good idea.
In terms of evicting the occupier process goes, you have no obligation or input here. You're not the immediate landlord, you have no standing to get involved.
You said you are "cancelling the contract" with the L2L company. There's the contract and then there's the tenancy. You could (intentionally or otherwise) ends the tenancy you have with the L2L without the L2L being able to comply with terms associated with end of tenancy, e.g. provide vacant possession back to you. In such case, you inherit the problem of having to evict the tenant. In theory and extra cost incurred can be sued for against the L2L, but refer to above about your chance of success.
I would suggest you seek professional advice.