Technology and functions Edit
The Doctor's Gladstone bag was a notable source for many objects for which the Doctor had use. (COMIC: Shark Bait, et al.) By one account, the Gladstone bag, much like the TARDIS, was bigger-on-the-inside, explaining how so many convenient items could be packed inside. However, this was revealed to be a dream being had by the Eighth Doctor, and although he also had one of the bags in his real TARDIS, it is hard to gauge how much of said account applied to the real Gladstone bag. (COMIC: The Land of Happy Endings) The Seventh Doctor's Gladstone bag seemed to have regular dimensions on the inside. (TV: Doctor Who) The Gladstone bag didn't always carry everything that the Doctor needed, as he once cursed himself for not packing it with water. (COMIC: Plague of the Black Scorpi)
The earliest account of the First Doctor using one of these bags was on a sand planet, where the TARDIS sunk into an underground cave. In order to pass a watery area in the cave, the Doctor pulled several pairs of flippers in order to allow them to swim across. When his companion and grandchild John was attacked by a giant crab, the Doctor stopped it with an electroliser. The bag was left in the water, and the Doctor left the planet without retrieving it. (COMIC: Shark Bait)
The Doctor brought another bag to a snowy planet, where he met Santa Claus. In order to help Santa Claus make toy TARDISes, he emerged a magic box which could shrink or enlarge objects. The Doctor again left the planet without the bag. (COMIC: A Christmas Story)
The Doctor then visited a desert planet with his grandchildren. He later cursed himself for not bringing water with them in the bag. Upon discovering a grouping of Coconut trees, the Doctor dropped the bag. He left the planet without retrieving it. (COMIC: Plague of the Black Scorpi)
At the end of his life, the Seventh Doctor was using a Gladstone bag as a toolbox. He pulled the Sonic screwdriver from the bag, using it to seal the Master's remains. Later, the Eighth Doctor used the bag while trying to jump-start the TARDIS. (TV: Doctor Who)
Behind the scenes Edit
Having first appeared in the TV Comic story Shark Bait, the Gladstone bag was invented by the writers of the comic as a "'catch all' saviour of all script dead ends".  In the days before such devices as the sonic screwdriver, the Gladstone bag allowed for quick resolutions to things in the way of the main characters, allowing for more to happen within the narrative.
While featuring in only three comic stories, the bag has since gone on to become a minor staple of the Doctor's accessories in other media, being used often in the BBC Eighth Doctor Adventures. As noted above, a Gladstone bag was also used as the base for the Seventh Doctor's toolbox, although it's unclear if the connection was purposeful on the part of the production team.
It made a notable return to comics during the Doctor Who Magazine celebration of TV Comics in the story The Land of Happy Endings, where it was used by the Eighth Doctor. The comic revealed many secrets about the bag, such as that it was bigger on the inside, but the ultimate twist of the story was that most of the narrative shown (and possibly any story to feature John and Gillian) was a dream inside the Doctor's mind. However, the Gladstone bag apparently also existed within the "real" TARDIS, again confusing how much of the story had been true.