Once in training room, press the key just under "Esc" on your keyboard to open the console.
Depending on your keyboard (QWERTY or AZERTY), it's
"~" or
"²".Then, just type
/g_gravity 100 and press
"Enter".
It will then be much more easier to finish all the training levels!!
Then, go back in console and type
/g_gravity 800 and press
"Enter" to revert the gravity to the default value (sorry for not mentionning this earlier).