You've mentioned two issues which are probably separate.
CPU Usage
What is using 100% of your 4 cores? MATE System Monitor or htop are two good ways to look at what is using your cores.
A GPU will only be able to ease the load on the CPU for certain tasks. This includes things like:
- video decoding (or encoding) - so watching youtube for example;
- applications that do a lot of image compositing and blending (i.e. OBS Studio);
- video games.
Memory usage
What applications are "vampiring" off your RAM? Again, MATE System Monitor and/or htop can guide you.
Whether a GPU is actually able to free up memory is not obvious and depends a lot on the situation (e.g. it's not always possible to free a 'buffer' on the system memory side even when the data has been copied over to the GPU memory to do some work on it).
NOTE: We've discussed elsewhere on this forum that Linux will often use a lot of RAM to ease the burden on your SSD (or HDD). See this post: What is using so much memory?