The 3GB number that you have is actually what is available to Windows to use. In fact, your system has 4GB of physical memory (it just can't use it all because of BIOS (etc.) requirements). The G33 chipset is capable of supporting up to 8GB of memory. Whether this motherboard will support 8GB or not is something that you will have to aks and have Gateway answer. I know that Intel's boxed G33-based motherboard products can support 8GB...
P.S. Doc is correct; if your board can take 8GB and you successfully upgrade it, you will have to install a 64-bit O/S in order to utilize the additional 4GB of memory.
I think is an Intel board, but if I understand correctly to the other comment, I can put 8gb of memory but it will be no difference because I have to install a new version of Windows, like Windows 7? Am I correct? or i will feel the difference between 3gbs that i have now and the 6 or 8 that i will put in the board, Thank you for the reply
You may be able to go to 8gb, or maybe not. You will need to confirm with Gateway.
Windows 7? What OS and architecture do you have currently? Windows XP, Windows Vista? 32 bit or 64 bit? As said before, to use more than 4gb of memory, you will need a 64-bit OS.
As far as the board being Intel, as I said before it may have been made by Intel for Gateway. In this case, Gateway needs to provide the relevant information.
Also, this is an old board. Spending time with questions and trying to configure this board may cost you more than purchasing a current board.