Im a bit confused here. Are gravel vacuum and siphon the same thing ?
Siphon is device (usually hose) used to get water out of fish tank.
A gravel vacuum has a special attachment that partially sucks up gravel removing lighter gunk while siphoning the water out.
So a gravel vacuum is a siphon with a special attachment on the end on it.
You can get one device that does both. As previously mention a syphon is a hose and you can add an extra attachment which is used for gravel. When you gravel vacuum you also take out water as well as all the poop on the bottom.
While both posts above are definitely correct, the vast majority of people will use the two terms interchangeably, IMO.
I have both - I use a manual siphon with a hand pump to remove water during a water change. It has a little plastic shower head-style attachment on the tank end that sucks up some waste, but it's not hugely effective for that, so I have also bought an electric Fluval Provac gravel cleaner which has a long kinda tube thingy on the tank end that you press into the gravel, where it removes the waste by passing the water through an internal filter, them passes out the 'recycled' water without the waste in it. You can attach a tube to the Fluval Provac thing to siphon out the water altogether when doing a water change, but that kinda over-complicates it for me, so I just stick to my manual siphon for my weekly water change and use the electric gravel cleaner once a month or so for a deeper clean