In this guide im going to teach you one of the most difficult trick in hl2dm named yo-yo. The peculiarity of hl2dm is the gravity gun, so your good aim and movement is not enough to be good at hl2dm, you should also master the gravity gun. As i said it is a very difficult trick for beginner, but If you will learn yo-yo, you will have no problems with gravity gun.
The main goal of yo-yo trick is to reset the timer of grenades or AR2 balls, but it also give you advantage in close ggun fights or in grabbing items from under the stairs on dm_lockdown. Probably you did not yet understand what i mean, don't worry and read next.
The common problem of ggun is you can't use it instantly for the second time right after you used it once, it have some delay (needs to be reloaded). You can "reload" ggun faster if you will change weapon to another one and then switch back to ggun: it will be ready to use again. So to make your ggun reload quickly you should switch it very fast. Some people double press lastinv or phys_swap binds. I personally prefer using 2 buttons to reload ggun, because it preserves you from making mistakes. Here are my binds:
I use WASD for movement and the closest buttons to reload ggun; q(i use melee weapon because it will never be out of ammo) and f (the closest button to my index finger).

In fact Yo-yo trick is as easy as 1, 2, 3, 4:
1. Attack1 (throw your item with ggun)
2. Switch to Another weapon
3. Switch Back to ggun
4. Attack 2 (grab the item again while it's still near you)

The easiest item to start practice with is ar2 ball. It is the slowest item. For other physic items here is the rule: the smaller the item is, the more difficult to yo-yo it. I recommend you to face a wall while practicing, so you won't lose the ball if you fail to recatch it.

Now watch the video for details.
GL&HF.