Is the meaning of "beat on" in the last sentence of The Great Gatsby only "we continue to advance" or something else? So we beat on, boats against the current, borne back ceaselessly into the past.
Certainly The Great Gatsby has as a theme that of repeating the past as Jay Gatsby draws upon it to rekindle his dream of reuniting with Daisy. But, in reality, Gatsby’s dream was behind him
...somewhere back in that vast obscurity beyond the city, where the dark fields of the republic rolled on under the night.
Gatsby’s dream, in all its idealism, asked too much.
Continuing this theme of repeating the past, in the last chapter, then, Nick Carraway calls upon the past as renewal rather than death. For, he realizes that the false, materialistic American Dream of the Jazz Age is dead, and the old values of America must be renewed by recalling the true idealism of the past,
It eluded us then, but that’s no matter—tomorrow we will run faster, stretch out our arms farther.… And one fine morning.…
Nick suggests that Americans can transcend and recreate [advancing, in a poetic sense] the past--"we beat on"--in order to find the true heartbeat of those early settlers who had an aesthetic contemplation of the wondrous new land of opportunity.