Only the host where the machine is installed can ping or see the VM on the network, returning results from the arp or telnet. But what I want is for another host to see this VM. The ultimate goal is to make this host connect, through the x2go client, to the VM with x2go server; but neither ping works, nor arp, nor telnet. An important detail is that the VM sees this host, but not the other way around. There is no firewall enabled either on the VM, nor I your host, nor on the other host.

What kind of virtualization engine did you use?
What are the operating systems on the VM host and guest? What versions?
What subnets are involved?
Exactly how did you set up the networking for the VM guest?
What are the IP addresses involved? (I presume they are all local, so that revealing them is not a security issue.)
What do your routing tables look like in the VM host and guest?
What does your command look like on the remote machine?

That is just to start, so that our next round of questions can be more pointed. Asking a question without providing information enough to understand the question is never likely to result in useful answers.

You say this:
then you say that:
This looks like a contradiction to me. Does the host see the VM or not? If not, my guess is that ip_forward is disabled. If yes, perhaps the VM is connected with NAT, which is the default for VirtualBox and libvirt/KVM.

But please provide context and answer Wpeckham's questions.

What kind of virtualization engine did you use? Virtual Box
What are the operating systems on the VM host and guest? What versions? VM host: Windows 10; Guest: Ubuntu 19.10
What subnets are involved? Only net 192.168.100.0/24
Exactly how did you set up the networking for the VM guest? Mode Bridge; promiscuous mode allow all
What are the IP addresses involved? host vm 192.168.100.7; vm 192.168.100.33; host x2goclient 192.168.100.3
What do your routing tables look like in the VM host and guest?

vm host: (windows)
dest 0.0.0.0 router 192.168.100.1 interface 192.168.100.7

guest: (ubuntu)
dest 0.0.0.0 router 192.168.100.1 options UG
dest 192.168.100.0 router 0.0.0.0 options U

What does your command look like on the remote machine?
in host x2goclient (192.168.100.3):
ping 192.168.100.33
telnet 192.168.100.33 1022
arp -a (return only 192.168.100.1 and 192.168.100.7)

GOOD start.

If the networking is set properly on the guest and in virtualbox, then only something on the VM host woudl block that traffic. Let us see if we can eliminate issues in the guest a bit first.

How did you set the networking within the guest? Was that set static, or dynamic (using dhcp perhaps)?
Can you display the result of the command on that guest? This will give us both address and subnet definition information that may help.

Have you turned on any listeners in the guest? At what ports do they listen? Can you verify that they listen on the external IP address and port as far as the guest networking shows?

I am not at all interested in arp table information at this time. I have found arp tables of little help and often misleading in recent months. The important thing is IP traffic, routing, listening, and firewalls. If you are not seeing traffic, there is going to be a reason why: one that we can find.

It is unfortunate that your host is running Win10, as Microsoft seems to decide you should not be able to do or see certain things: and the mix of what they allow, disallow, or hide can change without warning during even minor updates. For that reason I do not have a Win10 installation handy to help with troubleshooting. We may have to depend on others for those steps. I am confident that, if we persevere, we will be able to resolve the issue. When Win7 was in support I often ran several guests (one or two at a time) in bridged mode without seeing this issue.

The ip on the guest is configured as dynamic (using dhcp perhaps)

So it should be picking up the same network setup and nameserver (DNS) settings as your host, from the dhcp server.
And for the rest?

VM Host Windows -> dynamic
Host with x2goclient -> static

Okay, but: Can you display the result of the command
on that guest? This will give us both address and subnet definition information that may help.

Have you turned on any listeners in the guest?
At what ports do they listen?
Can you verify that they listen on the external IP address and port as far as the guest networking shows?