Gain or Chart ?

Which of the following is important for a day trader when considering a stock ?
% Gain from last closing price
or
Trend of Today's Chart

Trend is your friend. You can judge the market mood by Trend. GANN is useful for S&R levels.

Trend of Today's Chart alongwith the quantum of volume associated with it.

What is the importance of volume ?

Does it matter whether the stock belongs to Nifty, Jr Nifty or others ?

Volume is important as it is volume which moves the price up or down significantly. Personally, it doesn't matter whether a stock belongs to Nifty, Junior Nifty or Others as long as it has a good volume.

What is the reasonable time interval for checking the charts (5, 10, 15...30min) taking into consideration volatility of some stocks ?

Resonable time interval for checking charts differs from person to person. There is no hard and fast rule. It entirely depends on the trading mindset of the trader and the aspect of his profit booking. Some traders book profit fast with small increase in scrip value. They tend to look at smaller timeframe charts. Whereas some traders book profit slow but huge. They tend to look at longer time frames.

Your observation is great:thumb:. I am slowly but surely realizing the importance of volume.