As you well know, Chelsea cannot recruit new players in the next year. They were penalized by a transfer embargo after illegally recruiting underage players.
Lampard currently needs a new left back in his team.
The figure of Marcos Alonso is claimed to be incompatible with the scheme he is building. For that he needs a new left back to accompany Emerson Palmieri.
Gaya is considered to have shown a stable performance with Valencia. For that he is considered to be suitable to be part of the Blues squad.