LTM/GTM integration

We have an HA pair of LTMs in 2 data centers and are adding GTM. Do we configure GTM on one side of the HA pair in each site? Or if we were to put it on both, we'd have to pay for 4, correct?

I've done GTMs in the past but always on standalone devices. Never really thought conceptually how it would work when adding it to an HA pair.

Thanks!