Well, if you can find a working used NVIDIA GeForce GTX 670M video card (you must match P/N exactly), the replacement is straight forward. What I've seen as a problem with the age of the G75 series is how the plastic panels are getting very brittle. Disassembly and reassembly can cause cracks and breaks around some of the metal screw inserts. When going into one, have a good super glue nearby.
My G75VW DS73 3D US model is fully upgraded (223 BIOS, 32Gb memory, all SSD storage, Intel 7260 WiFi card, and Windows 10) and works very good. I believe I replaced the bottom case because of cracking about 2 years ago. Apart from that, I've repasted the CPU and GPU, every couple years, replace the battery every three years, even replaced the power brick a couple times, and clean the fans often.
I see replacement video cards are costing about $100, I would replace the card and make some upgrades while in there.
If you can, please run dxdiag.exe and confirm your model. See example below. I believe you have a French 3D model. If so, those video cards are very hard to find.
G75VW-9Z203V - Nvidia 3D Vision + Max HDD
CPU: Intel® Core™ i7-3610QM (2,30 GHz - 3,3 GHz)
Memory: 8GB (2x4GB), 1,600MHz
Display: 17,3" Full HD 1080p (1920 x 1080) LED backlit, Nvidia 3D Vision with Lightboost, 120Hz
Graphics: NVIDIA® GeForce™ GTX 670M 3GB GDDR5
HDD: 2x 1.5TB 7,200RPM (RAID 0 option)
ODD: DVD±RW DL / BD-ROM
LAN: Gigabit
Wi-Fi: 802.11n
Intel Wireless Display (WiDi): Yes
Bluetooth: Yes 4.0
DisplayPort: 1 x mini
Thunderbolt: No
Battery: 8 cell 5200mAh
Software: Windows® 7 Edition Home Premium 64 bit & Microsoft Office 2010 Starter
Good Luck
G752VSK, G75VW-3D, G51J, G1S
Homebuilt Windows Server