first you go to the town to the right where the grass gyme is and ask the old lady in one of the buildings near the pokemon center then she will give you tea and you take the tea to the guard
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You need to get around to Celadon (via Lavender Town), visit the Pokemon Mansion there and an old lady will give you a hot drink, take it to the guard and he will pass it round.
At the top of Celdon city , well there is a building there and right at the last floor you can go to some vending machines and buy water and give it to them
There is no blocked gate, just a thirsty guard who won't let you past them and get into Saffron city unless you get them a drink from the Celadon Dept. store from the vending machine. All four guards won't let you past, by the way but the good news is that you only need to buy one drink because they all share the same drink.
If you mean the guard by the elavator, you can't get passed him.
Go to the Silph Co. in celadon and get the sliph scope.