How can I display variable like standard gdb does?

[yayuwang0624]

I tried display i but it does not show the value of i in the following execution

[hugsy]

Use print

[yayuwang0624]

but it is a one-time print not a persistent display right?

[hugsy]

What do you mean by "persistent"? Do you want to make it part of the context in GEF?

[yayuwang0624]

yes.

like in the original gdb once you display a variable, it will be always printed after you execute next, continue etc.

can I also do this with gef?

[hugsy]

there are several ways but the easiest would be to use the memory watch command, or you can also create your own context pane.
both ways are documented, that should help you to start.