Work essentially wants me to do two jobs at once? How should I deal with this?

Hey everyone, I'll try to make this as understandable as possible and I hope it's not too confusing to read:

Tim is our department head.
I used to work on Gary's team and Tim now wants to move me to Ross' team.
I agreed because I've always found Gary very difficult to work with and I have quite liked Ross in all my interactions with him.

I'm working on a big project at the moment that I agreed to bring with me to Ross' team, as it's almost finished and I've been the one managing it from the start so it would be pointless to try and hand it over to someone else now.
The project is quite big and made up about 30% of the work I was doing on Gary's team.

However, Gary is insistent that I still need to action all the reports I used to do when I was on his team.
This is a bit crazy because there are a lot of those reports and they make up about 70% of the work I did on Gary's team.
So what Gary is essentially saying is he wants me to continue doing every single piece of work I was doing on his team, although I now work for Ross and have new work there.

I discussed this with Ross who told me not to worry - he agrees with me that Gary is being unreasonable and sent him an e-mail (with me cc'd) saying that I won't be continuing to run those reports going forward.
I was nice about it, even sharing documentation explaining how to run the reports and offering to help the next person who is assigned them.

But Gary is now demanding that I still run those reports.
Ross is currently on three weeks of annual leave and Gary keeps asking me to join 1-to-1 calls where he tries to get me to agree to run the reports.
I keep telling him I don't have capacity and he insists that I do.

His demenaour is a bit threatening to be honest - he's told me that my PR will suffer if I don't do this and he keeps saying that Tim agrees with him that I should have to run the reports.
I've asked him if we can communicate via e-mail instead so that Ross can read a record of our discussion when he gets back but then he says I'm being "aggressive" by refusing his phonecalls.

I tried talking to Tim directly about it but he just says things like "we'll work it out if it gets too stressful for you" and "do the reports for now and let's see how it goes" which isn't very helpful.

Is there anything I can do?
Tim doesn't really seem to be on my side, Gary isn't backing down, and Ross isn't back for three weeks.
I'm really scared they'll just force me to do two jobs, and I'm wary about going to HR because they exist to help the company - not me.

To be honest, I'd love to just resign and walk away from the whole mess but I just can't afford to do that right now.
Advice requested and gratefully received. x