GIMP Version: 2.8.10
Operating System: Windows
GIMP Experience: Beginner Level

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I have a Wacom intuos 2 that works fine in photoshop but brush opacity doesn't change at all with pressure in gimp. In preferences, the pen and eraser show up as installed. How do I tell gimp to have a brush change opacity with pressure etc?

You can change the dynamics

my brush dynamics were set to "pressure opacity" but it still didn't change opacity with pressure

Did you click the little button at the bottom of the dynamics with the little pencil, edit dynamics? You might be able to set up your pressure and opacity there.

You have to configured your tablet using, Edit>Preferences>Input Devices>Configure Extended Input Devices>Configure Input Devices, and set the "Mode" to "Screen".

Once this is done.
You need to set the "Brush Dynamics" to "Basic Dynamics". "Pressure Opacity" should also work, as long as the configuration "Mode" is set to "Screen". As seen in the image above.
This should allow you to use your "Pressure Stylus/Wacom" as it's intended.

the brush is set to basic dynamics and mode is screen . but edit dynamics is grayed out . And the eraser end on the pen is working as a brush. any ideas?

I'm not sure why this is so, but my Gimp also has grayed-out brush dynamics. I'm thinking that if the pen-tablet is used, this may be why the brush-dynamics are grayed-out.
I use my mouse most of the time and don't use brush-dynamics with the mouse.

Ok, I opened Gimp. I clicked the Dynamics button. The first five down from the top are not grayed out, whether I use the mouse or the pen. All the ones from basic dynamics down to the bottom are grayed out. Strange or what.

Yes...
I'm not sure whats going on, but when I need to use my Wacom tablet all works well, and when I use my mouse everything's okay for me, with the dynamics turned off.
So I'm not sure what to do about this issue.

I am talking about the dynamics editor, Everything works fine for me except most of the editor except the top five are grayed out.

Yes Molly, I understand what you're getting at, but I haven't any suggestions as to why or how to change this.
I'm stumped...

I noticed on your screen shot the other day that your Wacom and mine have a few different settings I have had mine for over three years so it isn't new.

I've had my Wacom for a little longer, but my computers is new.
My Wacom is a "MTE450 Bamboo".
It was purchased as a refurbished Wacom tablet. Which was considerably less expensive then buying a new one, and it works nicely.

Mine is a Wacom Bamboo Fun ... CTE 650K with mouse and pen included.

Nice!
My Wacom has a small, 5.8" x 3.7" active area, and doesn't come with a mouse, but that was okay by me.

Ten years late but here's a solution.
Simply delete the grayed out input devices 'WACOM Tablet Eraser' and 'WACOM Tablet Pressure Stylus' To open Input Devices: Edit > Input Devices. Make the changes then restart GIMP. Go back into Input Devices your stylus options will now, no longer, be grayed out. Reassign the inputs to 'Screen.' Happy Painting.

Actually i tried it and i cant do it, I have the same problem... and nothing is working lol, totally idk what to do. You can see my tries on screen below. Also the settings are working for eraser but not for brush...
Also my settings are changing when I am using mouse and a tablet pen... why? The pencill automatically has different brush type and size, should it be like that?