Gum is a tricky process because there are so many variables. This is true for other alt processes like cyanotype etc. as well, but with the pigment processes (gum, carbon, intaglio etc) the number of variables really explodes. What you end up is literally a million ways a print could be made, and every practitioner having their own unique approaches.
I can only recommend studying the contemporary masters - people like Bob Carnie, who has a few videos online, and if you can, get Calvin Grier's Gum Printing eBook. That one is really good because it not just explains how he does it, but it mostly goes into a couple of relevant areas of underlying theory. For instance, paper sizing is something he dedicates a number of pages to, starting with the concept of surface energy of the paper and then exploring several ways of sizing paper.
What happens here is that without sizing, you've got a lot of pigment staining. It so happens that carbon black pigment (which you are likely using in one form or another) is a heavily staining pigment that embeds itself between/on the paper fibers and won't entirely go away, even with prolonged washing. Sizing the paper seals the fibers off, making the staining issue less. But in this particular case, you ended up with a sizing layer that apparently has insufficient surface energy - i.e. it doesn't have sufficient affinity with the gum layer you put on top, which consequently mostly slides off during development.
I think Calvin used a formalin-hardened gelatin size for his gum prints, but I also recall that he applied a second coat (perhaps an acrylic gesso, or it might have been albumen - I'd have to check his book). Then again, he ended up using a different approach to developing the gum layer, adding some hydroxide to the gum and sensitizer, and then using development in accordance with pH. This resulted in more or less instantaneous development that was also perfectly controllable/consistent. As far as I know, he's the only one to do it that way. Other practitioners use an unmodified gum sensitizer and a lengthy soak, followed by very gentle mechanical action, usually by pouring water onto the print (see e.g. Bob Carnie's videos).
The few times I made gum prints I sometimes ended up with an adhesion that wouldn't withstand any form of mechanical action; any form of brushing or even pouring water onto the print would wash away the image. Other times (on the more successful attempts), I found that it actually took quite some brushing to get the image to appear and the image would be rather stable/resilient, but it also tended to be rather grainy. I remember using hardened gum as a paper sizing for those prints, but the paper I use was quite textured, which likely didn't help much.
In my experience, yes, this is actually one of the easier things to do, but the results I got were quite grainy as mentioned above. But with a high pigment load, you end up with pretty much a binary image that consists of paper white (plus perhaps some pigment stain...) and near black. For smoother results, and to get a nice and solid black, multiple layers are necessary. I have to admit I never really got beyond 3 layers, at which point further layers wouldn't adhere well anymore. I didn't spend much time troubleshooting that issue back then and moved on to other processes.
I'll alert
@Andrew O'Neill since he has done a lot of gum and he usually has useful insights to share. Perhaps a few other of our resident gum printers (although they are few and far between on this forum) might also chime in.