did you do any search on the internet before you asked? this is basic routing! if there are no overlapping addressing, then the configuration is straightforward. G1 needs to know the subnets for G3 are reachable through G2 and vica versa . this can be done by configuring status routes or by using a dynamic routing protocol - first setup each individual site G1, G2 G3 (G2, G3 allready done) - on G1 configure G2 as the next-hop for the networks of G3 - on G3 configure G2 as the next-hop for the networks of G1 details depend on the rest of your configuration.
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