Outstanding order in the Selling process in Degiro

A GTC order implies you set a limit order, you want to sell at a certain price, higher than what it was trading at.

There's nothing to do except watch your account. GTC is usually good for 90 days, not an indefinite time.

When you submit a GTC order, it goes on the order book in Price and Time priority. The order's duration varies from broker to broker.

Make sure to keep track of your open orders because if the price is reached, your order will be filled. That's no problem unless you manually filled your order prior to the execution of your GTC order.

If I sell my share with Order duration GTC (Good till cancelled), the order will became an Outstanding order.

Correct.

When it will be active to sell?

The order will be 'active' immediately. However it will be a limit order, which means that it will not execute unless the limit price is reached. Depending on how far away the limit price is from the market, it could be a short period of time or a long period of time, if at all.

Do I need to make any action to continue the selling process?

Generally no. The order will sit in the order book until the price is reached. However, some brokers will place a limit on the duration of GTC orders (such as 3 months), or if the exchange is updating their systems, they may cancel all the GTC orders on their exchange before the system change. Usually you will be notified of these types of events. It doesn't hurt to periodically check the state of your GTC orders.

Effectively GTC limit orders are no different from typical Day limit orders, with the exception that a Day limit order is cancelled at the end of the trading day whereas GTC orders are saved and re-inserted into the order book the next business day when the exchange comes back up for trading.