There is a single direct train daily from Poland to Lithuania.

https://ltglink.lt/en/kaunas-bialystok-kaunas

Global Pass is valid on this and on railways through the 3 Baltic states however it may not be the best option as routes are faiirly limited and local fares quite cheap, it may be better to use a mix of trains with local fares and the comprehensive network of bus links across the region.

This train from PL is very new-hitherto was only 1 on weekends/summertime.
Best info for global overview-as always: on seat61.com

NO trains however between LT and LV-there were in the past, cancelled since covid. Plus that those who insist on train-only for LV->EE face an hourlongs wait at some minor dead bordervillage-this wait is even longer as the direct bus would take.

I see you come from RO - a pass might be not a good idea if this is the major trip you plan to take-check WIZZair for a quick flite back and score cheap advance tix for to go to Bp/HU then to PL. IN PL there are variuous also quite cheap local passes, but restricted to companies.

Weekends: yes. Summer only: no, as I've told you a dozen times by now. You might try reading the information on seat61.com. There's been a daily train since 11 December, as announced months ago.

Thanks for all the info - I also found this daily train Warsaw-Vilnius in the LT site : https://ltglink.lt/en/vilnius-warsaw-krakow-vilnius  but it cannot be found on the Eurail ap. Is it bookable and are reservations needed?

The rail planner app and the Eurail/Interrail website are not reliable and should not be used to plan. Additionally, it seems that Lithuanian railways were late sending their timetables and they're not included in the app yet. The timetable is already included in the Deutsche Bahn planner, as a through train (but you will have to change in Mockava).

Reservations seem to be mandatory, at least for the IC train from Poland to Mockava, and should be available from a PKP ticket office, not online unfortunately.