Should i secure the capital gain in robo-advisor by withdraw it when it achieve a certain amount like $1,000, and invest the capital gain? So, the unit of the stock will increase. Continue below...

Assume u 1st invest $10k, when price increase to $11k. U sell $1k, 90.9units of stock. Now you left with 909.1units of stocks.

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Then you buy back 90.9unit again?

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It make no different actually. Your portfolio value will still remain $11k. Same number of units

Lets say you invested $10k in a robo-advisor portfolio. The portfolio grows +10% to $11k, so your capital gain is $1k. You sell $1k worth of your portfolio, your "unit" in that portfolio actually decrease. Lets say you immediately invest this $1k capital gain back to the portfolio, you are now buying at the new price and therefore get back the exact number of "unit" you have just sold. In the end, it still holds the same number of "unit" as you first started.