All I know about my old laptop is it is a Compaq. I can post a serial number if that is useful here.
There's no model number on the notebook?
How old is the old notebook?
Unfortunately, the BIOS will only tell how much RAM is installed, not what type. The Gateway uses DDR3 RAM. If the Compaq uses DDR2, don't even try. The slots are not even physically compatible. If you force the wrong type RAM in the slot, you will damage the slot (and likely the RAM too) permanently.
According to the specs and Crucial
for the Gateway, it supports up to 8GB DDR3 in a 2 x 4GB configuration. If me, I would just bite the bullet and buy all new 2 x 4GB to ensure total compatibility and dual channel operation.
Even if your Compaq is using DDR3, it is not likely it has the same specs as your Gateway RAM so no guarantee it is compatible with the Gateway motherboard/chipset, or the existing Gateway RAM, or that it will run in dual channel mode.
At the very least, pull the Compaq RAM. Determine if DDR2 or DDR3 as seen here
. If DDR2, there is no point in even thinking of trying it.
Before even thinking of touching the RAM in either computer, pull the batteries and unplug the chargers. Then touch bare metal inside the case to discharge any static in your body. Then touch the RAM in the edges - never the electrical contacts.