Is it a good idea to Harvest each month?

This depends on a few factors, one being the price of GAIN tokens on Decentralized and Centralized exchanges. When there is a premium for GAIN tokens on exchanges, it is better
to simply sell your GAIN tokens on an exchange as opposed to Harvesting, as you
will receive a better rate for your tokens.

Another factor is your time horizon for your investment. If you are a long term holder and believer of crypto as an asset class, it will behoove you to simply hold your GAIN tokens vs Harvesting them, which comes with some associated costs.