Back in the late seventies and early eighties most arcade and console games had no sort of copy protection whatsoever. Anyone with the proper hardware could copy an Atari 2600 cartridge. Of course the people with the proper hardware were so rare that they were almost non-existent. The same thing went for arcade games as well, the only thing protecting them was the fact that you had to build a circuit board for them, and that most of them had a few custom chips on them.
Bootlegs really started taking off in 1981. Scramble sold a zillion copies and then Konami/Stern started putting out other games that would run unmodified on a Scramble PCB. Scramble boards were getting converted over to the hot new titles left and right. Pretty soon they were modifying the software from other games to run on the Scramble hardware. Eventually they were making entire clone PCBs of hot games and selling them at discount prices. The manufacturers made this particularly easy with their total lack of copy protection. Most games even came with full detailed schematics.
Once you have the schematics and the data it becomes trivial for a dedicated bootlegger to copy the game. All one has to do is write a few extra routines to make up for the lack of custom chips and you are done. Namco might have had thousands and thousands of man hours invested in programming Galaga, but the bootlegger custom version could be ready to ship in a few weeks. Not to mention the fact that the bootlegger can just concentrate on the games that are already hits, something the legitimate developers are unable to do.
Gallag is a very common bootleg version of the arcade game Galaga.
Most arcade bootlegs are quickly hacked together and are prone to early failure, but not Gallag. Gallag boardsets are just as high quality as original Namco Galaga boards, which means that a lot of them have survived to the current day.
The only actual difference between "Galaga" and "Gallag" are the text on the title screen and the copyright text (which says "1 9 8 2" instead of "1981 NAMCO LTD").
The Gallag hardware is similar to the Galaga hardware, but it is not on an identical set of boards, but it will plug into the same wiring harness. You should always check any Galaga machine you are purchasing very carefully. I know from experience that ninety percent of the Galaga machines showing up at most arcade game auctions have Gallag or other bootleg boards inside them. The sellers stick the cheaper $100 bootleg boards in them because the auction bidder will probably never check inside. Then they later sell the real Galaga boards for $250 on eBay.
If you have a Gallag it can be changed to display the title "Galaga" by replacing a single EPROM on the main board. This modification has often already been done if you get a "Gallag" in a Galaga cabinet.