> I want to know if it is possible to configure and UCI engine like
> Crafty or GnuChess to play at a specific ELO level?
Neither of them are UCI, they are both xboard.
For UCI engines there might be an option to set through your
UCI-setup. The options in question are UCI_LimitStrength and
UCI_Elo. Anyway the UCI engine in question has to support
them.

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Kind regards,
Alexander Wagner