What did Sir Charles do while he stood by the gate in The Hound of the Baskervilles?
While Sir Charles Baskerville stood by the gate, waiting to meet a lady, he smoked.
We know that he smoked for at least five or ten minutes because he had dropped ash twice from his cigar.
Sir Charles was there to meet Laura Lyons, with whom he had fallen in love. She didn't really love him, though. She was using him.
She was in love with the man who called himself Jack Stapleton. She thought he would marry her if she got a divorce. He had her arrange to meet Sir Charles to ask for money.
Stapleton has Lyons miss the meeting, but since he knows Sir Charles will be at the gate, he can scare him to death with the hound.
In Ch.2 of Sir Arthur Conan Doyle's The Hound of the Baskervilles we learn that Sir Charles was in the habit of smoking a cigar before he went to bed. Dr. James Mortimer reads to Sherlock Holmes the article dated 14th May from the Devon County Chronicle,
“The facts of the case are simple. Sir Charles Baskerville was in the habit every night before going to bed of walking down the famous Yew Alley of Baskerville Hall. The evidence of the Barrymores shows that this had been his custom. On the 4th of May Sir Charles had declared his intention of starting next day for London, and had ordered Barrymore to prepare his luggage. That night he went out as usual for his nocturnal walk, in the course of which he was in the habit of smoking a cigar."
On that fateful night when Sir Charles was waiting at "the wicket-gate which leads on to the moor," for the arrival of Laura Lyons he was smoking a cigar for at least five or ten minutes. Dr. James Mortimer tells Holmes,
sir charles baskerville was smoking his pipe while waiting for lady.