Is working in a permissible advertising company - that includes a non-mandatory gambling game - lawful or unlawful?

If an employee's work is limited to handling permissible matters and does not aid in anything forbidden, then there is no harm in him working for the platform. The presence of a game does not prevent working to publish permissible advertisements on it. The decisive factor is the employee's work itself and the content of the advertisements he publishes. If they are permissible, then there is no harm in that work nor in what he earns from it.