The answer is that it varies in each Church. Technically one would keep 'counting up' from the Pentecost of the previous year (and, beginning in September, 'counting up' the Gospel readings from Luke) until one starts the Triodion for the next year.
In years where an early Pascha is followed by a late Pascha there is the gap (that you are asking about). The readings are repeated but the way this done can be slightly different between Greeks & Slavs and even among each group. For the Greeks, the Ecumencial Patriarch now simply publishes a list. [The Melkites appear to be nearly identical to this, with allowances for the different date for Pascha.] Among the Ruthenian Catholics it appears the the tendency is to count backwards from Pascha to the leave-taking of Theophany. The practical effect is that the readings from the weeks after Pentecost which would normally be skipped (due to Christmas and Theophany) are taken. But different local Churches do things differently. I've not done a study but my expectation is that most Orthodox and Greek Catholic Churches simply continue 'counting up' from Pentecost (Epistles) and from the Exaltation (Gospels) and when they run out they calculate enough of a repeat to take them to Zacchaeus and then to the Triodion cycle.
See the
lectionary page at this site and the
Greek Kanonion [
goarch.org] for more information. Father David Petras of the Ruthenians publishes a good
Typicon [
davidpetras.com] that provides a full lectionary.
